kernel - Fix sendfile() bugs / nginx

* Fix a sendfile() bug tickled by nginx.  When sending partial file bodies the
  nbytes test was improperly included the header bytes in its file body limit
  test.  This caused sendfile() to return 0 (complete success) but not
  actually output the entirety of the requested file segment.

* sendfile() now holds VM pages instead of wiring them.  Wiring the pages
  prevents truncation operations from succeeded and can further cause a
  kernel panic if the underlying file is deleted and the descriptor is
  closed while mbufs related to the sendfile() are still pending on the
  socket.

Massive-debugging-by: joris
ucom - Hack unref to fix infinite 1-second wait loop.

* ucom depended on the .tsw_* switch to decomission ports, which we do
  not use currently.  This was preventing the ucom from being properly
  dereferenced and resulted in an infinite 1-second wait loop on close.

* Move the unref elsewhere to hopefully solve the problem.