Like matchers, targets should use a module-like register/lookup system. This
replaces the brittle switch statements we had before.
The only behavior change is supporing IPT_GET_REVISION_TARGET. This makes it
much easier to add IPv6 redirect in the next change.
Updates #3549.
PiperOrigin-RevId: 334469418
iptables sockopts were kludged into an unnecessary check, this properly
relegates them to the {get,set}SockOptIP functions.
PiperOrigin-RevId: 328395135
