treat external storage allocated by the mbuf layer the same as

treat external storage allocated by the mbuf layer the same as
external storage attached to an mbuf anywhere else. this means it
uses MEXTADD to wire it up to the mbuf, and it relies on the ext_free
and ext_arg bits in the header to call the right free function
against the right pool.

M_CLUSTER gets renamed to M_EXTWR. the type field in MEXTADD gets
reused as a flags field so anything attaching storage to an mbuf
can say if it is writable or not.

ok claudio@