These functions were mostly identical, except for iptables being enabled
by default (unless explicitly disabled by config).
Rewrite the function to a enabledIptablesVersions, which returns the list
of iptables-versions that are enabled for the controller. This prevents
having to acquire a lock twice, and simplifies arrangeUserFilterRule, which
can now just iterate over the enabled versions.
Also moving this function to a linux-only file, as other platforms don't have
the iptables types defined.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
