MFC r272978: 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 r273008: Remove stale comments.
MFC r273029:
Report physical block size for file-backed LUNs, using vattr.va_blocksize.
MFC r273038:  Add support for READ DEFECT DATA (10/12) commands.

SPC-4 r2 allows to return empty defect list if the list is not supported.
We don't reallu support defect data lists, but this suppresses some errors.
MFC r273046:
Don't confuse frontend with zero length data moves, just return immediately.
MFC r273072:  Add LBPERE mode bit definition.