Make use of the NULL macro

Use this constant everywhere when referring to a null pointer instead of
casting 0 to various types of pointers. Created using following semantic
patch:

    @@
    type type;
    @@

    - (type *)0
    + NULL

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

src/calendar.c

@@ -130,7 +130,7 @@ calendar_date_thread (void *arg)
       calendar_update_panel (&win[CAL]);
     }
 
-  return (void *)0;
+  return NULL;
 }
 
 /* Launch the calendar date thread. */