shared: add ip_protocol_{from|to}_tcp_udp helpers

Thin wrappers of ip_protocol_{from|to}_name targeting IPPROTO_TCP and
IPPROTO_UDP only.
Used to parse IP protocol configuration restricted only to TCP and UDP,
e.g. in SocketBind{Allow|Deny}= unit property.

These helpers are inspired by af_{from|to}_ipv4_ipv6 and potentially
extendable with other IP protocols if there is a use-case to expose
them.

src/shared/ip-protocol-list.c

@@ -65,3 +65,13 @@ int parse_ip_protocol(const char *s) {
 
         return i;
 }
+
+const char *ip_protocol_to_tcp_udp(int id) {
+        return IN_SET(id, IPPROTO_TCP, IPPROTO_UDP) ?
+                ip_protocol_to_name(id) : NULL;
+}
+
+int ip_protocol_from_tcp_udp(const char *ip_protocol) {
+        int id = ip_protocol_from_name(ip_protocol);
+        return IN_SET(id, IPPROTO_TCP, IPPROTO_UDP) ? id : -EINVAL;
+}

src/shared/ip-protocol-list.h

@@ -4,3 +4,6 @@
 const char *ip_protocol_to_name(int id);
 int ip_protocol_from_name(const char *name);
 int parse_ip_protocol(const char *s);
+
+const char *ip_protocol_to_tcp_udp(int id);
+int ip_protocol_from_tcp_udp(const char *ip_protocol);