bridge(4), lagg(4) etc. and make use of function pointers and
pf_proto_register() to hook carp into the network stack.
Currently, because of the uncertainty about whether the unload path is free
of race condition panics, unloads are disallowed by default. Compiling with
CARPMOD_CAN_UNLOAD in CFLAGS removes this anti foot shooting measure.
This commit requires IP6PROTOSPACER, introduced in r211115.
Reviewed by: bz, simon
Approved by: ken (mentor)
MFC after: 2 weeks
