During fragment reassembly, mbuf chains with packet headers were

During fragment reassembly, mbuf chains with packet headers were
created.  Add a new function m_removehdr() do convert packet header
mbufs within the chain to regular mbufs.  Assert that the mbuf at
the beginning of the chain has a packet header.
found by Maxime Villard in NetBSD; from markus@; OK claudio@