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/sigs.c

@@ -83,7 +83,7 @@ sigs_set_hdlr (int sig, void (*handler)(int))
   sigemptyset (&sa.sa_mask);
   sa.sa_handler = handler;
   sa.sa_flags = 0;
-  if (sigaction (sig, &sa, (struct sigaction *)0) == -1)
+  if (sigaction (sig, &sa, NULL) == -1)
     {
       ERROR_MSG (_("Error setting signal #%d : %s\n"),
                  sig, strerror (errno));