New post_crash_dump_hook

Run it from crash_dump().  Useful to move core files out of the way,
alert deities by e-mail, and so forth.

src/server/main.c

@@ -340,6 +340,7 @@ crash_dump(void)
 	    return;
 	}
     }
+    run_hook(post_crash_dump_hook, "post-crash-dump");
     logerror("Crash dump complete");
 #endif
 }