basic: introduce socket_protocol_{from,to}_name()

And use them where they can be applicable.

src/basic/socket-protocol-list.h

@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+const char *socket_protocol_to_name(int id);
+int socket_protocol_from_name(const char *name);
+
+int socket_protocol_max(void);