Example usage:

    $ on-demand-ssh-tunnel 20110 110 -p2222 user@ssh-server 

When something connects to 127.0.0.1:20110 on-demand-ssh-tunnel will
start ssh subprocess

    ssh -v -L random-tunnel-port:127.0.0.1:110 -n -p2222 user@ssh-server

to tunnel communication to POP3 server accessible on port 110 (on
server).