Use local port forwarding on '-L' command line option.

Use the `ssh -L` option to forward a random local port to the remote
cups port and execute `lp`/`lpr` locally.
Local port forwarding starts background session
Use (set -x; ...) subshell also for local forwards
Add '-C' option to specify remote cups port.

Furthermore:
 - rename variable `port` => `cupsport`
 . in function `random_port` make variable `port` local