Drop group privileges before dropping user privileges when execing

This ensures that the saved-set-gid is also set, although this shouldn't
matter in practice since we always call execve() soon after, which
overwrites the saved-set-gid with the effective gid anyway.

source/screen.c

@@ -1620,8 +1620,8 @@ extern	Window	*BX_create_additional_screen (void)
 			static char geom[32];
 			int i;
 
-			setuid(getuid());
 			setgid(getgid());
+			setuid(getuid());
 			setsid();
 
 			/*