Introduce libseat_set_log_level

The default level is SILENT. log_init no longer takes an initial log
level (so that calls to libseat_set_log_level prior to log_init work
correctly).

libseat/libseat.c

@@ -37,7 +37,7 @@ struct libseat *libseat_open_seat(struct libseat_seat_listener *listener, void *
 		return NULL;
 	}
 
-	log_init(LIBSEAT_LOG_LEVEL_SILENT);
+	log_init();
 
 	char *backend_type = getenv("LIBSEAT_BACKEND");
 	struct libseat *backend = NULL;