Encode the Loopback Address

POSIX. Implement void fill_loopback(struct sockaddr_in *sa, int port) — zero the struct, set family AF_INET, address to INADDR_LOOPBACK (network byte order), and port (network byte order). Then implement int decode_port(const struct sockaddr_in *sa) returning the host-order port. NULL sa returns -1 from decode_port and does nothing in fill.