Cleanup joinable threads on termination

Always invoke pthread_join() when we blow up a thread via
pthread_cancel() (avoid zombie threads).

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calendar.c

@@ -145,7 +145,10 @@ void
 calendar_stop_date_thread (void)
 {
   if (calendar_t_date)
-    pthread_cancel (calendar_t_date);
+    {
+      pthread_cancel (calendar_t_date);
+      pthread_join (calendar_t_date, NULL);
+    }
 }
 
 /* Set static variable today to current date */