Added IPv6 code into free-port.pl

Added IPv6 code into free-port.pl via the command line -6 flag.
Added documentation to code about -6 flag

Added documentation to code about -6 flag and added a gracious empty line to clearly show the -6 option code vs the existing code.
Merge pull request #73 from mahtin/patch-1

Added IPv6 code into free-port.pl