MFC r267905:
Add READ BUFFER and improve WRITE BUFFER SCSI commands support.

This gives some use to 512KB per-LUN buffers, allocated for Copan-specific
processor code and not used.  It allows, for example, to test transport
performance and/or correctness without accessing the media, as supported
by Linux version of sg3_utils.
MFC r267906:
Allow MODE SENSE commands through Write Exclusive persistent reservation,
as required by SPC-4.

Report that fact in persistent reservation capabilities.
MFC r267933:
Simplify statistics calculation.

Instead of trying to guess size of disk I/O operations (it just won't work
that way for newly added commands, and is equal to data move size for old
ones), account data move traffic.  If disk I/Os are that interesting, then
backends have to account and provide that information.

Block backend already exports the information about disk I/Os via devstat,
so having it here too is excessive.
MFC r268204:
Use separate memory type M_CTLIO for I/Os.

CTL allocate large amount of RAM.  This change give some more stats.
MFC r268392:
Do not return statuses for aborted iSCSI commands.
MFC r268283:
Improve readability of XML generated by CTL_LUN_LIST.