provide m_dup_pkt() for doing fast deep mbuf copies with a specified alignment

if a physical interface receives a multicast/broadcast packet and
has carp interfaces on it, that packet needs to be copied for
reception by each of those carp interfaces.

previously it was using m_copym2, but that doesn't respect the
alignment of the source packet. this meant the ip header in the
copies were aligned incorrectly for the network stack, which breaks
strict alignment archs.

m_dup_pkt lets carp specify that the payload needs an ETHER_ALIGN
adjustment, so the ip header inside will be aligned correctly.

reported and tested by anthony eden who hit this on armv7
i reproduced the problem on sparc64 and verified the fix on amd64
and sparc64
ok mpi@ mikeb@ deraadt@

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