Optimise and extend the port command.

1. greatly improved the efficiency of the the -u and -a modes, by gathering port state once instead of invoking netstat for each port (O(n^2)).
2. reduced code duplication.
3. added new command -f to pick a random free port.
4. improved the -s command so that report port state in exit as well, and also print the user cgroup postfix.
5. migrated from 'netstat' (obsolete according to the man page) to 'ss'.
6. 'ss' output is handled by 'awk' as better suitable for tabular format.