MFC r278037: CTL LUN mapping rewrite.

MFC r278037: CTL LUN mapping rewrite.

Replace iSCSI-specific LUN mapping mechanism with new one, working for any
ports.  By default all ports are created without LUN mapping, exposing all
CTL LUNs as before.  But, if needed, LUN mapping can be manually set on
per-port basis via ctladm.  For its iSCSI ports ctld does it via ioctl(2).
The next step will be to teach ctld to work with FibreChannel ports also.

Respecting additional flexibility of the new mechanism, ctl.conf now allows
alternative syntax for LUN definition.  LUNs can now be defined in global
context, and then referenced from targets by unique name, as needed.  It
allows same LUN to be exposed several times via multiple targets.

While there, increase limit for LUNs per target in ctld from 256 to 1024.
Some initiators do not support LUNs above 255, but that is not our problem.

Relnotes:	yes
Sponsored by:	iXsystems, Inc.

git-svn-id: svn://svn.freebsd.org/base/stable/10@279002 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f