Allow USERNAME to be specified on the command line.

There are some systems that have usernames of the form DOMAIN\username,
which causes an invalid escape character to be inserted.  I was going to
add some escaping, but decided it would be best to just outright set the
value, since I don't want the DOMAIN portion in there anyways.
Remove proto as an include directory.

The proto directory was removed in 0ef90c13b72b74928bfb3c183c7a5bd7240b51ad.
Remove sizeof(int) < 4 related code
cleanup os_unixx.h
inline the rest of os_unixx.h into os_unix.c
Broken build on 32 bit: Fix -Wshorten-64-to-32.

Problem:

[ 48%] Building C object src/CMakeFiles/nvim.dir/os/mem.c.o
/Users/eliseo/projects/os/neovim/src/os/mem.c:9:32: error: implicit
conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long long') to 'long_u' (aka
      'unsigned long')
            [-Werror,-Wshorten-64-to-32]
              return uv_get_total_memory() >> 10;
                ~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~^~~~~

Solution:

Avoid conversion. Make function return proper uint64_t.
Make users of the function accomodate the value if too big for them.
Broken build on 32 bit: Fix -Wtautological-constant-out-of-range-compare.

Problem:

[ 51%] Building C object src/CMakeFiles/nvim.dir/os/rstream.c.o
/Users/eliseo/projects/os/neovim/src/os/rstream.c:237:24: error: comparison of constant
      9223372036854775807 with expression of type 'size_t' (aka 'unsigned long') is always true
      [-Werror,-Wtautological-constant-out-of-range-compare]
  assert(rstream->fpos <= INT64_MAX);
         ~~~~~~~~~~~~~ ^  ~~~~~~~~~
/usr/include/assert.h:93:25: note: expanded from macro 'assert'
    (__builtin_expect(!(e), 0) ? __assert_rtn(__func__, __FILE__, __LINE__, #e) : (void)0)
                        ^

Solution:

Assertion temporarily disabled.
TODO: Review types so that assertion can be used.
Use portable format specifiers: Case %lld - plain - sprintf.

Fix uses of plain "%lld" within sprintf():
- Replace "%lld" with "%" PRId64.
- Cast corresponding argument to (int64_t).
Use portable format specifiers: Case %lld - localized - sprintf.

Fix uses of localized "%lld" within sprintf():
- Replace _("%lld") with _("%" PRId64).
- Cast corresponding argument to (int64_t).
- In *.po files, replace "%lld" with "%<PRId64>".
Use portable format specifiers: Remove branching with LONG_LONG_OFF_T.

After previous commits, branching code by using macro LONG_LONG_OFF_T
becomes unneccesary. Remove it.
Use portable format specifiers: Case %ld - plain - sprintf.

Fix uses of plain "%ld" within sprintf():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).
Use portable format specifiers: Case %ld - localized - sprintf.

Fix uses of localized "%ld" within sprintf():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).
Use portable format specifiers: Case %ld - plain - fprintf.

Fix uses of plain "%ld" within fprintf():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).
Use portable format specifiers: Case %ld - plain - smsg.

Fix uses of plain "%ld" within smsg():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).
Use portable format specifiers: Case %ld - localized - smsg.

Fix uses of localized "%ld" within smsg():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).
Use portable format specifiers: Case %ld - plain - vim_snprintf.

Fix uses of plain "%ld" within vim_snprintf():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).
Use portable format specifiers: Case %ld - localized - vim_snprintf.

Fix uses of localized "%ld" within vim_snprintf():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).
Use portable format specifiers: Case %ld - localized - vim_snprintf_add.

Fix uses of localized "%ld" within vim_snprintf_add():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).
Use portable format specifiers: Adapt EMSGN/emsgn to use (int64_t).
Use portable format specifiers: Case %ld - plain - EMSGN.

Fix uses of plain "%ld" within EMSGN():
- Replace "%ld" with "%" PRId64.
- No argument cast needed. EMSGN() will take care of that.