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
9f4c7ba460