pulse-server: handle out-of-files better

When we don't have enough files to accept the connection, clear the
_IN flag so that we don't try to accept if over and over again.
When a client disconnects, set the flag again so that we try to
accecpt new connections again.

See #1305

src/modules/module-protocol-pulse/internal.h

@@ -198,6 +198,7 @@ struct server {
 	struct spa_source *source;
 	struct spa_list clients;
 	uint32_t n_clients;
+	uint32_t wait_clients;
 	unsigned int activated:1;
 };