Added unidirectional flags -S and -R

For example, forwarding UDP stream from mmpeg over SSH:

    udptunnel -R -s 9990 127.0.0.1/7777
    ffmpeg -re -i blah.mkv -f mpegts 'udp://127.0.0.1:7777?pkt_size=188'

    udptunnel -S -c 127.0.0.1/9990 127.0.0.1/7778
    ffplay -i 'udp://127.0.0.1:7778'