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/fd-util.h

@@ -79,8 +79,6 @@ int pack_fds(int fds[], size_t n);
 int fd_validate(int fd);
 int same_fd(int a, int b);
 
-void cmsg_close_all(struct msghdr *mh);
-
 bool fdname_is_valid(const char *s);
 
 int fd_get_path(int fd, char **ret);