Retire MT_HEADER mbuf type and change its users to use MT_DATA.

Retire MT_HEADER mbuf type and change its users to use MT_DATA.

Having an additional MT_HEADER mbuf type is superfluous and redundant
as nothing depends on it.  It only adds a layer of confusion.  The
distinction between header mbuf's and data mbuf's is solely done
through the m->m_flags M_PKTHDR flag.

Non-native code is not changed in this commit.  For compatibility
MT_HEADER is mapped to MT_DATA.

Sponsored by:	TCP/IP Optimization Fundraise 2005