main: Fix color schemes for abstract_ui

- Set 't_Co' to 256 at startup. The value can be changed by the user for
  compatibility with terminals that are less capable.
- `has('gui_running')` will return 1 if at least one rgb UI is attached.

Even though these changes are hacky, they are necessary to make the transition
to the new UI architecture smoother.

src/nvim/ui.c

@@ -114,6 +114,16 @@ void ui_write(uint8_t *s, int len)
     free(tofree);
 }
 
+bool ui_rgb_attached(void)
+{
+  for (size_t i = 0; i < ui_count; i++) {
+    if (uis[i]->rgb) {
+      return true;
+    }
+  }
+  return false;
+}
+
 /*
  * If the machine has job control, use it to suspend the program,
  * otherwise fake it by starting a new shell.