Do some refactoring and minor cleanups of the thread_single() code in
preparation for the global stop commit.

Move the code to weed suspended or sleeping threads into the
appropriate state, into the helper weed_inhib().  Current code already
has deep nesting and hard to follow [1].

Add currently useless helper remain_for_mode(), which returns the
count of threads which are allowed to run, according to the
single-threading mode.

In thread_single_end(), do not save curthread into local variable, it
is unused after, except to find curproc.

Remove stray empty line.

Requested by:	avg [1]
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Current reaction of the nfsd worker threads to any signal is exit.
This is not correct at least for the stop requests.  Check for stop
conditions and suspend threads if requested.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week