patch 9.0.0206: redraw flags are not named specifically

Problem:    Redraw flags are not named specifically.
Solution:   Prefix "UPD_" to the flags, for UPDate_screen().
diff --git a/src/os_unix.c b/src/os_unix.c
index 07762f4..2f00614 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -1228,7 +1228,7 @@
 	// back to a sane mode. We should redraw, but we can't really do that
 	// in a signal handler, do a redraw later.
 	after_sigcont();
-	redraw_later(CLEAR);
+	redraw_later(UPD_CLEAR);
 	cursor_on_force();
 	out_flush();
     }