Stage 1/many: mbuf/cluster accounting rewrite and mbuf allocator rewrite.

Stage 1/many: mbuf/cluster accounting rewrite and mbuf allocator rewrite.

Implement 'new' style mbuf m_ext buffers where the callbacks are called with
a custom argument instead of with (buf, size).  Old style m_ext buffers have
to jump through loops to properly track reference counters in a manner that
is tightly integrated with the old mbuf/cluster KVA allocation array.

The old clustering methodology is going to have to be converted to the new
methodology before we can rip out the old mbuf allocator, plus we want to
use the new mechanism anyway because it is a whole lot cleaner and because
it's easier to import driver code (aka NDIS) from FreeBSD-5 with the new
mechanism.

This commit still supports the old mechanism via M_EXT_OLD, and all
existing function-based M_EXT's have been converted to use M_EXT_OLD.  The
old mechanism (and M_EXT_OLD) will be removed in later stages.