Fix warnings: cursor_shape.c: parse_shape_opt(): Garbage value: FP.

Problem: Assigned value is garbage or undefined @ 187.
         http://neovim.org/doc/reports/clang/report-7b7d61.html#EndPath.

Diagnostic: False positive.
Rationale : `colonp`, must be `>= modep, or null` by `vim_strchr`
            postcondition. At this point we also it's not null and it's
            not equal to `modep`, by previous code. So, it must be
            `> modep`.
Resolution: Assert `colonp > modep`.
Fix warnings: screen.c: redraw_asap(): Various (6): MI.

Problems: Argument with 'nonnull' attribute passed null @ 277.
          http://neovim.org/doc/reports/clang/report-9c3614.html#EndPath

          Result of operation is garbage or undefined @ 281.
          http://neovim.org/doc/reports/clang/report-45efbf.html#EndPath

          Argument with 'nonnull' attribute passed null @ 306.
          http://neovim.org/doc/reports/clang/report-ffb84f.html#EndPath

          Result of operation is garbage or undefined @ 311.
          http://neovim.org/doc/reports/clang/report-d04333.html#EndPath

          Argument with 'nonnull' attribute passed null @ 315.
          http://neovim.org/doc/reports/clang/report-786819.html#EndPath

          Uninitialized argument value @ 328.
          http://neovim.org/doc/reports/clang/report-2a5506.html#EndPath

Diagnostic: Multithreading issues.
Rationale : All reported problems can only occur if accesed globals
            change state while executing function, which could only
            happen in a multithreaded environment.
Resolution: Use local variables.
            Note that this change alters function semantics, as now
            function only depends on global values at entry time.
            This shouldn't be a problem, though, as new semantics should
            be in fact better.
Fix warnings: screen.c: win_line(): Dead initialization: HI.

Problem: Dead initialization @ 3477.
         http://neovim.org/doc/reports/clang/report-94b736.html#EndPath

Diagnostic: Harmless issue.
Rationale : `len` is assigned a new value just some lines below. So,
            this just seems something due to old-style variable
            declarations.
Resolution: We could just remove initialization, but prefer moving
            declaration down to point of initialization.
Fix warnings: screen.c: win_line(): Dead assigment: HI.

Problem: Dead assigment.
         http://neovim.org/doc/reports/clang/report-7362ba.html#EndPath

Diagnostic: Harmless issue.
Rationale : `boguscols` is in fact unread by downstream code.
Resolution: Comment out. This is preferred here over just removing the
            line because involved logic is complex, and future readers
            of this code could find this extra knowledge useful to
            understand what the code is doing.
Fix warnings: screen.c: showmode(): Dead assignment: HI.

Problem: Dead assignment @ 7535.
         http://neovim.org/doc/reports/clang/report-19a5cd.html#EndPath

Diagnostic: Harmless issue.
Rationale : `length = msg_col;` is unconditionally executed after this.
Resolution: Remove assignment.
Fix warnings: screen.c: draw_tabline(): Dead assignment: HI.

Problem: Dead assignment @ 7711.
         http://neovim.org/doc/reports/clang/report-835eb6.html#EndPath

Diagnostic: Harmless issue.
Rationale : `scol` is only used within `FOR_ALL_TABS` body, which
            assigns another value to `scol` at the beginning of each
            iteration. If `FOR_ALL_TABS` body would not execute (no
            tabs) nothing harmful would happen, as code following
            `FOR_ALL_TABS` doesn't use `scol`.
Resolution: Remove.
Fix warnings: ops.c: do_join(): Nonnull violation: FP.

Problem: Argument with 'nonnull' attribute passed null @ 3540.
         http://neovim.org/doc/reports/clang/report-fc14e0.html#EndPath.

Diagnostic: False potitive.
Rationale : `count` should be >= 2 by function precondition.
Resolution: Assert precondition.
Fix warnings: ops.c: cursor_pos_info(): Various (2): MI.

Problems: Result of operation is garbage or undefined @ 5087.
          http://neovim.org/doc/reports/clang/report-2e3118.html#EndPath

          Result of operation is garbage or undefined @ 5149.

Diagnostic: Multithreading issues.
Rationale : All reported problems can only occur if accesed globals
            change state while executing function, which could only
            happen in a multithreaded environment.
Resolution: Use local variables (copy globals on entry).
            Note that this change alters function semantics, as now
            function only depends on global values at entry time.
            This shouldn't be a problem, though, as new semantics should
            be in fact better.
Fix warnings: regexp.c: skip_regexp: Np dereference: FP.

Problem: Derefence of null pointer @ 1208.
         http://neovim.org/doc/reports/clang/report-24b5ca.html#Path10

Diagnostic: False positive.
Rationale : Error is reported to happen  if after `if (*newp == NULL) {`
            body, `*newp` continues being NULL, and false branch of
            following `if (*newp != NULL)` is taken. Now, `vim_strsave`
            cannot return NULL, so error cannot happen.
Resolution: Remove dead code (leftover since OOM refactors).
Fix warnings: regexp.c: br_regcomp(): Np dereference: MI.

Problem: Dereference of null pointer @ 1312.
         http://neovim.org/doc/reports/clang/report-b1d09a.html#EndPath

Diagnostic: Multithreading issue.
Rationale : Suggested error path contains two succesive calls to
            `regnext(scan)`, first of which returning nonnull, the
            second one returning null. This can only occur if global
            `reg_toolong` accesed in `regnext()` changes between the
            calls.
Resolution: Use local variable to cache first `regnext(scan)` result.
            Note that this change alters function semantics, as now
            function only issues one call instead of two, reusing the
            result for the second time.
            This shouldn't be a problem, though, as new semantics should
            be in fact be better.
Fix warnings: regexp.c: match_with_backref(): Nonnull violation: FP.

Problem: Argument with 'nonnull' attribute passed null @ 5632.
         http://neovim.org/doc/reports/clang/report-041a0e.html#EndPath.

Diagnostic: False positive.
Rationale : `p = reg_getline(clnum)` above should not be null, because
            `clnum` starts at `start_lnum` and only increments after
            that.
Resolution: Assert p not null.
Fix warnings: edit.c: ins_compl_next_buf(): Np dereference: FP.

Problem    : Dereference of null pointer @ 3234.
Diagnostic : False positive.
Rationale  : `wp` is local static, so maintains value between calls.
             First time function is called for a given flag will have
             `buf == curbuf`, implying `wp` initialization.
Resolution : Assert variable always having been initialized.
Fix warnings: edit.c: ins_compl_get_exp(): Np dereference (2): FP.

Problems   : Dereference of null pointer @ 3615.
             Dereference of null pointer @ 3764.
Diagnostic : False positives.
Rationale  : `ins_buf` is local static, so maintains value between calls.
             This function will be called first when `compl_started` is
             false, and in that case it initializes `ins_buf`. After
             that, it can be called multiple times with `compl_started`
             true, where `ins_buf` will be updated but not to null.
             So, when arriving to both points, `ins_buf` should never be
             null.
Resolution : Assert `ins_buf` at both points.
Fix warnings: edit.c: mb_replace_pop_ins(): Unitilialized arg: FP.

Problem    : Uninitialized argument value @ 6296.
Diagnostic : False positive.
Rationale  : Error occurs if n <= 1. That's not possible because
             n >= 1 due to `MB_BYTE2LEN` postcondition and n != 1
             because we are in the else branch.
Resolution : Assert n > 1.
Fix warnings: edit.c: replace_do_bs(): Garbage value: MI.

Problem    : Assigned value is garbage or undefined @ 6359.
Diagnostic : Multithreading issue.
Rationale  : Problem only occurs if global `State` changes while
             function is executing.
Resolution : Use local copy of global in function.
Fix warnings: edit.c: ins_bs(): Garbage result: MI.

Problem    : Result of operation is garbage or undefined @ 7460.
Diagnostic : Multithreading issue.
Rationale  : Problem occurs if any of globals `enc_utf8`, `p_deco is
             modified while function is executing.
Resolution : Use local copy of globals.
Fix the nvim-clipboard help instructions

Fixes #1407
build: default to using -O2 for release builds

The -O3 optimization level can often lead to dangerous (and sometimes
incorrect) optimizations being performed.  So let's use a level that's
more stable.
memory: implement xmemrchr()
rstream: Expose rstream_read_ptr and _available.

Needed for accessing this information without direct access to the
rstream_buffer.