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/os_unix.c

@@ -175,13 +175,6 @@ void mch_exit(int r)
   exit(r);
 }
 
-#ifndef SEEK_SET
-# define SEEK_SET 0
-#endif
-#ifndef SEEK_END
-# define SEEK_END 2
-#endif
-
 #define SHELL_SPECIAL (char_u *)"\t \"&'$;<>()\\|"
 
 /// Does wildcard pattern matching using the shell.