Unlink the control socket on clean shutdown

Previously, the behaviour was to unlink any control socket sat where we
wanted to open ours.  This would make us lose control of running servers
if we happened to collide accidentally.  With this patch, the new
process will abort() if there is a control socket squatting on the
path we want, and unlink it when it closes.

This means that an unclean shutdown will leave a dangling, unattached
control socket which will block a restart, but that's a better option
than intentionally cutting off running servers.

src/util.c

@@ -18,18 +18,17 @@ void error_init(void)
 	pthread_key_create(&cleanup_handler_key, free);
 }
 
-void error_handler(int fatal __attribute__ ((unused)) )
+void error_handler(int fatal)
 {
 	DECLARE_ERROR_CONTEXT(context);
 	
-	if (!context) {
-		/* FIXME: This can't be right - by default we exit()
-		 * with a status of 0 in this case.
-		 */
-		pthread_exit((void*) 1);
+	if (context) {
+		longjmp(context->jmp, fatal ? 1 : 2 );
+	} 
+	else {
+		if ( fatal ) { abort(); }
+		else { pthread_exit((void*) 1); }
 	}
-		
-	longjmp(context->jmp, fatal ? 1 : 2 );
 }