Improve and document `ctladm portlist` subcommand.

Make this subcommand less FC-specific, reporting target and port addresses
in more generic way.  Also make it report list of connected initiators in
unified way, working for both FC and iSCSI, and potentially others.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272978 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f