mbufs: simplify mtags

[Maxime Villard <max%m00nbsd.net@localhost>]

I would like to simplify the mtag API. Basically, remove m_tag_first() and
m_tag_next(), and remove the 't' argument from m_tag_delete_chain() and
m_tag_find(). None of that is used. Then replace m_tag_delete_nonpersistent()
by m_tag_delete_chain(), both are the same.

Generally speaking mtags are bad, because slow to create/remove; few users
exist, and we don't want a big (~unused) API for something that we would
like network components not to do in the first place.

After that I would like to merge uipc_mbuf2.c into uipc_mbuf.c, initially
they were separate because the former had been imported from the outside,
but nowadays there is no point separating the two.