Windows: use $PATHEXT to find executables in path

Windows: include <io.h>

is_executable_in_path() searches for executables in $PATH, but on
Windows executable files have extensions available in the environment
var $PATHEXT. This commit changes is_executable_in_path() to append
those extensions to the filename.

This patch diverges from standard Vim, in that Vim only checked for
the given filename if it already has an extensions. This one always
checks for the given filename.

Unix systems must have this header but Windows does not have it at all.

Since src/nvim/os/unix_defs.h includes <unistd.h> without the guard
in order to avoid including this in the numerous places we would
need <unistd.h> on Unix we just include src/nvim/os/os.h which will pull
in <unistd.h> for us.

if_csope.c uses _open_osfhandle so include the necessary header.
Futher we need <io.h> for read/write/close/lseek.

See: https://msdn.microsoft.com/en-us/library/bdts1c9x.aspx

It's not present on Windows; see the discussion in #4024.

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

Among other things, this includes:
- lies about command.com
- references to pcterm

S_IFLNK is not defined on Windows but libuv defines it for us.

- ROOT_UID's comment is misleading, as it's always defined to 0.

- SEEK_{SET,END} & O_NOFOLLOW should already be defined on Unix-like
  systems in <stdio.h> and <fcntl.h>, respectively.  In any case,
  neither of those #ifdef blocks should be in the middle of source files.

- The S_IS{LNK,DIR,...} macros should only be undefined on Windows.