Initially done in 28e59cb22, but does not appear to be necessary
anymore.

Uses UV_EEXIST directly, just like UV_ENOENT.

libuv users are only supposed to directly include uv.h.  In v1.21.0, all
the uv-*.h headers were renamed to uv/*.h, which caused the unit tests
to fail with

    [123/125] Generating post/uv-errno.h
    FAILED: test/includes/post/uv-errno.h
    cd «SRCDIR»/src/neovim/build/test/includes && /usr/bin/clang -std=c99 -E -P «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h -I/usr/include -I/usr/include -o «SRCDIR»/neovim/build/test/includes/post/uv-errno.h
    «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h:1:10: error: 'uv-errno.h' file not found with <angled> include; use "quotes" instead
    #include <uv-errno.h>
             ^~~~~~~~~~~~
             "uv-errno.h"

The intention of the file is to extend libuv's error constants with more
values used by the unit tests.  This can just as easily be achieved
without poking into pseudo-private header files.

POSIX.1-2008[1] says that the latter should be used, and all of our
supported platforms would seem to support this scheme, apparently even
Windows[2].

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fcntl.h.html
[2]: https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx

Replaced old unit tests for errno with libuv error codes UV_ENOENT
and UV_EEXIST (for os_open and os_getperms).

Added libuv include path to test/includes compiler calls - needed
to get hold of libuv headers.

Use preprocessor trick proposed by @mahkoh to import 'defines' like
S_IRUSR.