- Create kern.ipc.sendfile namespace, and put the new "readhead" OID
  there as "kern.ipc.sendfile.readahead".
- Push all nsfbuf related tunables into MD code. Don't move them
  to new namespace in favor of POLA.

Reviewed by:	scottl
Approved by:	re (gjb)
Consistently use the same value to indicate exclusively-held and
shared-held locks for all the primitives in lc_lock/lc_unlock routines.
This fixes the problems introduced in r255747, which indeed introduced an
inversion in the logic.

Reported by:	many
Tested by:	bdrewery, pho, lme, Adam McDougall, O. Hartmann
Approved by:	re (glebius)