Unmapped mbufs for review

[John Baldwin]

Netflix has a set of local changes in their tree that they have been using
in production for a long while that add support for mbufs whose data are
unmapped pages of RAM.  The in-kernel TLS changes depend on these changes,
but they can also be useful standalone as they can allow for much shorter
mbuf chains in socket buffers when transmitting large amounts of data via
sendfile.  The vast majority of the work has been done by folks at Netflix,
and most of that by Drew Gallatin.  I'm just helping with reviewing it and
shepherding the patches into the tree.

There is a review open at https://reviews.freebsd.org/D20616

As I noted in the first comment, I will probably split this up into a few
separate commits, but I think it is probably easier to review as a large
patch so that you can see the end-to-end from sendfile down to a driver
(cxgbe in this case).

-- 
John Baldwin