Although UNIX is a registered trademark of The Open Group, it doesn't
really matter whether we refer to these systems as UNIX, Unix, or
Unix-like. So, for consistency, refer to them collectively as Unix.

Related:
http://www.greens.org/about/unix.html
http://www.unixica.com/html/unixunix.html

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.

os_file_is_readonly() in its current form is equivalent to
!os_file_is_writable(). This does not appear to be a bug, because Vim's
use of check_file_readonly() (which we changed to os_file_is_readonly())
is equivalent to !os_file_is_writable() in every case.

os_file_is_readonly() also fails this test:

    returns false if the file is non-read, non-write

A more useful form would define behavior under these cases:

  - path is executable (but not writable)
  - path is non-existent
  - path is directory

But there is no reason for os_file_is_readonly() to exist, so remove it.