Misc. macro cleanup

- 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.

src/nvim/vim.h

@@ -27,9 +27,6 @@ Error: configure did not run properly.Check auto/config.log.
 # endif
 #endif
 
-/* user ID of root is usually zero, but not for everybody */
-#define ROOT_UID 0
-
 
 /* Can't use "PACKAGE" here, conflicts with a Perl include file. */
 #ifndef VIMPACKAGE
@@ -232,15 +229,6 @@ enum {
 /* Size in bytes of the hash used in the undo file. */
 #define UNDO_HASH_SIZE 32
 
-#ifdef HAVE_FCNTL_H
-# include <fcntl.h>
-#endif
-
-
-#ifndef O_NOFOLLOW
-# define O_NOFOLLOW 0
-#endif
-
 /*
  * defines to avoid typecasts from (char_u *) to (char *) and back
  * (vim_strchr() and vim_strrchr() are now in alloc.c)