patch 9.1.1060: Vim always enables 'termguicolors' in a terminal

Problem:  Vim always enables 'termguicolors' in a terminal, even
          when not wanted (after v9.1.1054)
Solution: Respect `:set notermguicolors` in vimrc file

fixes: #16538
fixes: #16539
closes: #16540

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/option.c b/src/option.c
index 653b87c..3753c4a 100644
--- a/src/option.c
+++ b/src/option.c
@@ -4296,6 +4296,7 @@
 #  endif
 	    !has_vtp_working())
     {
+	p_tgc_set = TRUE;
 	p_tgc = 0;
 	return e_24_bit_colors_are_not_supported_on_this_environment;
     }
@@ -4320,6 +4321,7 @@
     term_update_palette_all();
     term_update_wincolor_all();
 # endif
+    p_tgc_set = TRUE;
 
     return NULL;
 }