Use curthread instead of the FIRST_THREAD_IN_PROC for vnlru and syncer,

Use curthread instead of the FIRST_THREAD_IN_PROC for vnlru and syncer,
when applicable.

Aquire Giant slightly later for vnlru.

In the syncer, aquire the Giant only when a vnode belongs to the
non-MPsafe fs.

In both speedup_syncer() and syncer_shutdown(), remove the syncer thread from
the lbolt sleep queue after the syncer state is modified, not before.

Herded by:	attilio
Tested by:	Peter Holm
Reviewed by:	ups
MFC after:	1 week