Optimise handling of select() returning <= 0.

Before, we immediately retried select() if it returned -1 and errno is EAGAIN
or EINTR, and if it returned 0 it would check for network events even if we
know there are none.  Now, if -1 or 0 is returned we skip checking network
events, but we do check for timer and signal events.

src/net.c

@@ -374,11 +374,10 @@ int main_loop(void) {
 				dump_connections();
 				return 1;
 			}
-
-			continue;
 		}
 
-		check_network_activity(&readset, &writeset);
+		if(r > 0)
+			check_network_activity(&readset, &writeset);
 
 		if(do_purge) {
 			purge();