MINOR: listener: use the protocol's ->rx_resume() method when available

Instead of calling listen() for IPPROTO_TCP in resume_listener(), let's
call the protocol's ->rx_resume() method when defined, which does the same.
This removes another hard-dependency on the fd and underlying protocol
from the generic functions.

src/listener.c

@@ -408,9 +408,8 @@ int resume_listener(struct listener *l)
 		goto end;
 	}
 
-	if (l->rx.proto->sock_prot == IPPROTO_TCP &&
-	    l->state == LI_PAUSED &&
-	    listen(l->rx.fd, listener_backlog(l)) != 0) {
+	if (l->state == LI_PAUSED && l->rx.proto->rx_resume &&
+	    l->rx.proto->rx_resume(&l->rx) <= 0) {
 		ret = 0;
 		goto end;
 	}