MFC r328337:

Add missing SPDX tags for ctld(8).

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Eliminate unused headers.

Fix error handling.

MFC after: 1 month
Coverity CID: 1249768
Sponsored by: The FreeBSD Foundation

Add basic iSNS client to the iSCSI target.

This makes ctld(8) register its iSCSI targets and portals on configured
iSNS servers to allow initiators find them without active discovery.

Fetching of allowed initiators from iSNS is not implemented now, so target
ACLs still should be configured manually.

Reviewed by: trasz@
MFC after: 1 month
Sponsored by: iXsystems, Inc.