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_join (calendar_t_date, NULL);
+    }
 }
 
 /* Set static variable today to current date */

src/io.c

@@ -2927,7 +2927,10 @@ void
 io_stop_psave_thread (void)
 {
   if (io_t_psave)
+    {
       pthread_cancel (io_t_psave);
+      pthread_join (io_t_psave, NULL);
+    }
 }
 
 /*

src/notify.c

@@ -188,7 +188,10 @@ void
 notify_stop_main_thread (void)
 {
   if (notify_t_main)
+    {
       pthread_cancel (notify_t_main);
+      pthread_join (notify_t_main, NULL);
+    }
 }
 
 /*