socket-util: add more careful parsers for AF_VSOCK cid/port

Let's handle the magic CIDs, and filter out invalid ports.

src/basic/socket-util.h

@@ -374,6 +374,9 @@ int socket_get_mtu(int fd, int af, size_t *ret);
 
 int connect_unix_path(int fd, int dir_fd, const char *path);
 
+int vsock_parse_port(const char *s, unsigned *ret);
+int vsock_parse_cid(const char *s, unsigned *ret);
+
 /* Parses AF_UNIX and AF_VSOCK addresses. AF_INET[6] require some netlink calls, so it cannot be in
  * src/basic/ and is done from 'socket_local_address from src/shared/. Return -EPROTO in case of
  * protocol mismatch. */