MT_TAG removal

[Sam Leffler]

http://people.freebsd.org/~sam/mt_tag.patch

This set of changes eliminates the use of MT_TAG "pseudo mbufs", replacing 
them mostly with packet tags (one case is handled by using an mbuf flag since 
the linkage between "caller" and "callee" is direct and there's no need to 
incur the overhead of a packet tag).

I've tested all these changes except for the PACKET_TAG_IPFORWARD stuff.

Before I can commit this stuff I need to do more testing (help welcome) and 
need to evaluate the cost of doing multiple m_tag_find calls in the IP packet 
processing paths.  I suspect we'll want to make this call first check if any 
tags exist before calling the code that does the actual list search.

I'm looking for code review and general feedback.  I especially like the way 
this cleans up divert sockets (in particular in the handling of fragments).

	Sam