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)

runtime/syntax/nosyntax.vim

@@ -24,9 +24,6 @@ augroup END
 
 if exists("syntax_on")
   unlet syntax_on
-else
-  " only used when starting, to disable the default syntax enable
-  let syntax_off = 1
 endif
 if exists("syntax_manual")
   unlet syntax_manual