options: Default t_Co to 256.

This commit doesn't change any behavior, only moves the init out of main.c We
_could_ move some initialization from tui.c:terminfo_start to an earlier phase,
in order to avoid mis-reporting 't_Co' during startup. But this will be messy,
and gains very little: TERM=linux works "good enough" as long as we correct t_Co
in tui.c:terminfo_start (c5b02d5a7).

src/nvim/option.c

@@ -4404,6 +4404,7 @@ bool get_tty_option(char *name, char **value)
 
   if (is_tty_option(name)) {
     if (value) {
+      // XXX: All other t_* options were removed in 3baba1e7.
       *value = xstrdup("");
     }
     return true;