Remote and local adv lock servers might de-synchronize (the added comment

Remote and local adv lock servers might de-synchronize (the added comment
explains the plausible scenario), resulting in EDEADLK returned on the
local registration attempt.  Handle this by re-trying the local op [1].

On unmount, local registration abort is indicated as EINTR, abort the nlm
call as well.

Reported and tested by:	pho
Suggested and reviewed by:	dfr (previous version, [1])
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (delphij)