Improve assertion that an ignored or blocked signal is not delivered.

Split two conditions into separate asserts.  Print additional details,
like the signal number and action value.

Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/kern/kern_sig.c

@@ -3030,8 +3030,10 @@ postsig(int sig)
 		/*
 		 * If we get here, the signal must be caught.
 		 */
-		KASSERT(action != SIG_IGN && !SIGISMEMBER(td->td_sigmask, sig),
-		    ("postsig action"));
+		KASSERT(action != SIG_IGN, ("postsig action %p", action));
+		KASSERT(!SIGISMEMBER(td->td_sigmask, sig),
+		    ("postsig action: blocked sig %d", sig));
+
 		/*
 		 * Set the new mask value and also defer further
 		 * occurrences of this signal.