fd-util: move cmsg_close_all() to socket-util.[ch]

This call only makes sense on sockets, it's not a generic fd concept,
hence let's move this over.

src/basic/socket-util.h

@@ -337,3 +337,5 @@ int vsock_get_local_cid(unsigned *ret);
 int netlink_socket_get_multicast_groups(int fd, size_t *ret_len, uint32_t **ret_groups);
 
 int socket_get_cookie(int fd, uint64_t *ret);
+
+void cmsg_close_all(struct msghdr *mh);