Add cbt protocol, to be able to mitigate some DDoS attacks
(MODULES-450) Enable rule inversion

iptables has many rule arguments that may be inverted by prefixing with
an exclamation mark. This commit enables inversion for most every
property currently in the firewall provider that supports inversion by
prefixing the value with a bang+space.

Array elements must have all array elements prefixed with a bang+space
otherwise a warning will be raised, as it would look confusing to negate
a single value and then have iptables negate all of them.
Remove stray print
Changes to firewall readme