Don't allow undefined references under Linux.

Many other systems expect this already, but on Linux the default is to
allow them.  Let's turn that off.
Tell cmake to use `-isystem` third-party includes

This adds the `SYSTEM` parameter to `include_directories`, which will tell cmake
to use `-isystem` instead of `-I` for specifying include directories. One
advantage is that compilers won't emit warnings for included files that belong
to dependencies.
Introduce nvim namespace: Move files.

Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.
Introduce nvim namespace: Fix build process.

- Leave src as include dir (for includes to recognize 'nvim/' prefix).
- Change subdirectory from src to src/nvim.
- Fix msgpack generation.
- Fix some other paths to new locations.
Introduce nvim namespace: Fix relative includes.

Problem:  Some newly introduced files used includes relative to the
          current file, both of the form `include "../XXX.h"` and
          `include "XXX.h"`.
          Preferred form is relative to include root (src/ in our case).
Solution: Change includes to preferred form.
Note:     This is also done to ease next commit (prepend 'nvim/ to all
          project-local includes).
Introduce nvim namespace: Fix project-local includes.

Prepend 'nvim/' in all project-local (non-system) includes.
Introduce nvim namespace: Fix unmasked strings.h issue.

Problem:  Now that nvim/strings.h is correctly namespaced, an issue
          that had been masked until now arises:

          When compiling, we get a lot of errors because of everywhere
          the functions in nvim/strings.h are used, there's no include
          to import them.

          But, how could this compile and work previously, then? It
          turns out that:
          - In every such case, we are also including vim.h, which in
            turn includes os_unix_defs.h.
          - os_unix_defs.h includes <string.h> and also <strings.h> in
            some systems (e.g. OSX).
          - Build had been modified previously to (even when importing
            system headers), prefer equally-named local ones. That was
            in fact done as a previous attempt to solve the same issue
            we are trying to solve another way now.

          So, we were including our "strings.h" as a side-effect of
          including <strings.h> through "vim.h" --> "os_unix_defs.h".

Solution: Correctly include "nvim/strings.h" in every file needing it.
Introduce nvim namespace: Fix unit tests.

Point cimports to new locations.
Introduce nvim namespace: Fix functional tests.

Fix some paths to new locations:
- Makefile: Fix nvim binary path.
- test49  : 459: Fix nvim binary path.
- test89  :  62: Fix testdir root.
                 Fix corresponding expected result.
- test105 :  10: Testing fnamemodify to reduce path to use ~ prefix.
                 Fix faked home directory.
                 Fix corresponding expected result.
             26: Testing fnamemodify with r modifier.
                 Fix out-of-project prefix removal.
                 Fix corresponding expected result.
Introduce nvim namespace: Fix clint.

- Fix path to clint-checked files.
- Fix mechanism to calculate define guard names.
Introduce nvim namespace: Fix localization.

- Fix executable path.
- Make po file title similar as others.
Introduce nvim namespace: Fix contrib youcompleteme.

- Fix instructions to install youcompleteme configuration.
Introduce nvim namespace: Fix define guards.

Change define guards from NEOVIM_XXX_H to NVIM_XXX_H:
- Change header files.
- Change clint correct guard name calculation.
s/GA_EMPTY/GA_EMPTY_INIT_VALUE/

I'm about to implement a `GA_EMPTY` macro that checks if the GA is empty or not.
Replace ga->ga_len > 0 checks with !GA_EMPTY(ga)

Used Coccinelle to perform the changes

```diff
@@
expression E;
@@

<...
(
- E.ga_len > 0
+ !GA_EMPTY(&E)
|
- E->ga_len > 0
+ !GA_EMPTY(E)
)
...>
```

`spatch --in-place --sp-file ga_empty.cocci <C_FILE>`
Replace ga->ga_len == 0 checks with GA_EMPTY(ga)

Used Coccinelle to perform the changes

@@
expression E;
@@

<...
(
// E.ga_len == 0 is isomorphic to !E.ga_len
- E.ga_len == 0
+ GA_EMPTY(&E)
|
- E->ga_len == 0
+ GA_EMPTY(E)
)
...>
Replace if (ga->ga_len) with if (!GA_EMPTY(ga))

Used Coccinelle to perform the changes

```diff
@@
expression E;
statement S;
@@

(
- if (E.ga_len) S
+ if (!GA_EMPTY(&E)) S
|
- if (E->ga_len) S
+ if (!GA_EMPTY(E)) S
)
```
Remove hardcoded type names from msgpack-gen.lua

Except for the `Error *` type, all parameter types are valid identifiers, so
reuse that LPeg rule.
Use `uintptr_t` as the hashtable type in helpers.c
Refactor API to use one integer type: int64_t

This should make the API simpler, and int64_t is enough to represent any integer
value we might need.

Range checks should be done inside the API functions, that way we can modify the
types of the actual fields/variables modified by the API without changes to the
API prototypes.