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/misc1.c

@@ -2729,13 +2729,6 @@ void fast_breakcheck(void)
   }
 }
 
-#ifndef SEEK_SET
-# define SEEK_SET 0
-#endif
-#ifndef SEEK_END
-# define SEEK_END 2
-#endif
-
 /*
  * Get the stdout of an external command.
  * If "ret_len" is NULL replace NUL characters with NL.  When "ret_len" is not