startup: Avoid VimL global. Introduce TriState enum.

- `syntax_on` is documented. Rather than introduce a new undocumented
  VimL global `g:syntax_off`, use a module-local flag.
- Rename "maybe" functions to follow style guidelines (use standard
  module prefix)

src/nvim/main.c

@@ -334,11 +334,10 @@ int main(int argc, char **argv)
 
   // If using the runtime (-u is not NONE), enable syntax & filetype plugins.
   if (params.use_vimrc != NULL && strcmp(params.use_vimrc, "NONE") != 0) {
-    // Do ":filetype plugin indent on".
-    maybe_enable_filetype();
-    // Enable syntax (sources syntax/syntax.vim, which calls `:filetype on`).
-    do_cmdline_cmd("if !exists('syntax_off') | syntax on |"
-                   "else | unlet syntax_off | endif");
+    // Does ":filetype plugin indent on".
+    filetype_maybe_enable();
+    // Sources syntax/syntax.vim, which calls `:filetype on`.
+    syn_maybe_on();
   }
 
   /*