Forward declare struct thread so that this header doesn't depend on its

Forward declare struct thread so that this header doesn't depend on its
own namespace pollution/compatibility cruft.

Removed the main part of the pollution.  All clients have been converted
to either not depend on getting old locking interfaces from this new
locking header, or usual case to get it from another header (typically
vnode.h, where declaring old loccking interfaces is less bogus because
vnode.h uses them internally).