Revert ROOT_UID removal

It helps clarify intent for those unaware of root's UID.

see https://github.com/neovim/neovim/pull/4015#discussion_r49822371

src/nvim/option.c

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

src/nvim/shada.c

@@ -3082,8 +3082,8 @@ shada_write_file_nomerge: {}
       // viminfo file that the user can't read.
       FileInfo old_info;
       if (os_fileinfo((char *)fname, &old_info)) {
-        if (getuid() == 0) {
-          if (old_info.stat.st_uid != 0
+        if (getuid() == ROOT_UID) {
+          if (old_info.stat.st_uid != ROOT_UID
               || old_info.stat.st_gid != getgid()) {
             const uv_uid_t old_uid = (uv_uid_t) old_info.stat.st_uid;
             const uv_gid_t old_gid = (uv_gid_t) old_info.stat.st_gid;

src/nvim/vim.h

@@ -39,6 +39,8 @@ Error: configure did not run properly.Check auto/config.log.
 
 #define MAX_TYPENR 65535
 
+#define ROOT_UID 0
+
 #include "nvim/keymap.h"
 #include "nvim/macros.h"