mbuf: add mc_split() that works on two struct mchain

[glebius]

Summary

It preserves tail points and all length/memory accounting, so that caller
doesn't need to do any extra traversals. It doesn't respect M_PKTHDR but
it may be improved if needed. It respects M_EOR, though. First consumer
will be the new unix(4) SOCK_STREAM and SOCK_SEQPACKET.

[tuexen]

sys/sys/mbuf.h

1778–1779

If adding mc_concat is intended, mention it in the commit message. Right now it looks like it slipped in the diff...

[markj]

sys/kern/uipc_mbuf.c

1125

This should assert that M_PKTHDR isn't set in head.

[markj]

sys/kern/uipc_mbuf.c

1145

Do we prefer m_align() in new code?

[glebius]

Review from Mark.

[glebius]

Mention mc_concat() in commit message.

[glebius]

Utilize mc_first, mc_last, etc.