Tidy up the login code; no functional changes.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit e2e58f80cc8a6e6632b043d08a1cc05ef3ccdcd6)
Make the initiator-name and initiator-portal checks a little nicer.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit 904ee8f9378b694958e470ffc7d0ae7fc8ad8f95)
Improve ctld.conf example.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit cbbca2de4dfaf610c857bebc91cd6029026797ad)
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.
Ticket:		#6368

(cherry picked from commit b91222fa4fd4a5dab2de697378099454a393c295)

Conflicts:
	usr.sbin/ctld/ctl.conf.5