[RFC] re-add has('mac') and has('macunix')

They were removed but they can be handy to separate some things to do at
startup in a .vimrc for example.

In vanilla vim:

eval.c:

```c
#ifdef MACOS
	"mac",
#endif
#if defined(MACOS_X_UNIX)
	"macunix",
#endif
```

vim.h

```c
/*
 * MACOS_CLASSIC compiling for MacOS prior to MacOS X
 * MACOS_X_UNIX  compiling for MacOS X (using os_unix.c)
 * MACOS_X       compiling for MacOS X (using os_unix.c)
 * MACOS	 compiling for either one
 */
#if defined(macintosh) && !defined(MACOS_CLASSIC)
# define MACOS_CLASSIC
#endif
#if defined(MACOS_X_UNIX)
# define MACOS_X
# ifndef HAVE_CONFIG_H
#  define UNIX
# endif
# ifndef FEAT_CLIPBOARD
#  define FEAT_CLIPBOARD
#  if defined(FEAT_SMALL) && !defined(FEAT_MOUSE)
#   define FEAT_MOUSE
#  endif
# endif
#endif
#if defined(MACOS_X) || defined(MACOS_CLASSIC)
# define MACOS
#endif
#if defined(MACOS_X) && defined(MACOS_CLASSIC)
    Error: To compile for both MACOS X and Classic use a Classic Carbon
#endif
/* Unless made through the Makefile enforce GUI on Mac */
#if defined(MACOS) && !defined(HAVE_CONFIG_H)
# define FEAT_GUI_MAC
#endif
```