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/option.c

@@ -837,9 +837,10 @@ set_option_default (
     } else {  /* P_BOOL */
       *(int *)varp = (int)(intptr_t)options[opt_idx].def_val[dvi];
 #ifdef UNIX
-      /* 'modeline' defaults to off for root */
-      if (options[opt_idx].indir == PV_ML && getuid() == ROOT_UID)
-        *(int *)varp = FALSE;
+      // 'modeline' defaults to off for root
+      if (options[opt_idx].indir == PV_ML && getuid() == 0) {
+        *(int *)varp = false;
+      }
 #endif
       /* May also set global value for local option. */
       if (both)