TUI: rework background-color detection

- Like Vim, use set_option_value() followed by reset_option_was_set().
- Do not use set_string_default(), so the default is predictable.
  This affects `:set bg&`.
- Wait until end-of-startup (VimEnter) to handle the response. The
  response is racey anyways, so timing is irrelevant. This allows
  OptionSet to be triggered, unlike during startup.

src/nvim/globals.h

@@ -83,10 +83,10 @@ EXTERN struct nvim_stats_s {
   int64_t redraw;
 } g_stats INIT(= { 0, 0 });
 
-/* Values for "starting" */
-#define NO_SCREEN       2       /* no screen updating yet */
-#define NO_BUFFERS      1       /* not all buffers loaded yet */
-/*                      0          not starting anymore */
+// Values for "starting".
+#define NO_SCREEN       2       // no screen updating yet
+#define NO_BUFFERS      1       // not all buffers loaded yet
+//                      0          not starting anymore
 
 /*
  * Number of Rows and Columns in the screen.