Dtrace: enablings on defunct providers prevent providers from unregistering

Merge change from illumos:

1368 enablings on defunct providers prevent providers from unregistering

We try to address some underlying differences between the Solaris
and FreeBSD implementations: dtrace_attach() / dtrace_detach() are
currently unimplemented in FreeBSD but the new code from illumos
makes use of taskq so some adaptations were made to dtrace_open()
and dtrace_close() to handle them appropriately.

Illumos Revision:	r13430:8e6add739e38

Reference:
https://www.illumos.org/issues/1368

Reviewed by:	gnn
Tested by:	Fabian Keil
Obtained from:	Illumos
MFC after:	3 weeks