Feature: add support for 'protocol' option

Feature: add support for 'protocol' option

The protocol option adds support for mass adding things like '-p tcp' in front of every rule.
This reduces repetition of the -p * commands.

The 'protocol' option accepts any single parameter or the reserved parameter 'none'.
Using 'none' will turn off the protocol option.
Other than 'none', this poses no restrictions on what the protocol may be.
Instead, that is left to iptables.