Migrate the sendfile_sync structure into a public(ish) API in preparation
for extending and reusing it.

The sendfile_sync wrapper is mostly just a "mbuf transaction" wrapper,
used to indicate that the backing store for a group of mbufs has completed.
It's only being used by sendfile for now and it's only implementing a
sleep/wakeup rendezvous.  However, there are other potential signaling
paths (kqueue) and other potential uses (socket zero-copy write) where the
same mechanism would also be useful.

So, with that in mind:

* extract the sendfile_sync code out into sf_sync_*() methods
* teach the sf_sync_alloc method about the current config flag -
  it will eventually know about kqueue.
* move the sendfile_sync code out of do_sendfile() - the only thing
  it now knows about is the sfs pointer.  The guts of the sync
  rendezvous (setup, rendezvous/wait, free) is now done in the
  syscall wrapper.
* .. and teach the 32-bit compat sendfile call the same.

This should be a no-op.  It's primarily preparation work for teaching
the sendfile_sync about kqueue notification.

Tested:

* Peter Holm's sendfile stress / regression scripts

Sponsored by:	Netflix, Inc.