Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r248983:
Dtrace: enablings on defunct providers prevent providers from unregistering

Merge change from illumos:

1368 enablings on defunct providers prevent providers from unregistering

Illumos Revision: 13430:8e6add739e38

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

Reviewed by: gnn
Tested by: Fabian Keil
Obtained from: Illumos

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

Update vendor/illumos/dist to pre libzfs_core state (dtrace and ctf part)
illumos-gate revision 13742:b6bbdd77139c

Obtained from: ssh://anonhg@hg.illumos.org/illumos-gate