Use proper term in debug messages.

MFC after:	1 month
Sponsored by:	FreeBSD Foundation
Improve code a little; no functional changes.

Sponsored by:	The FreeBSD Foundation
Rework session termination in iSCSI target to actually wait
for any outstanding commands to be properly aborted by CTL.
Without it, in some cases (such as files backing the LUNs
stored on failing disk drives), terminating a busy session
would result in panic.

Reviewed by:	mav@ (earlier version)
Sponsored by:	The FreeBSD Foundation
Fix typo and rename macro KDB_SYS_FLAG to KBD_SYS_FLAG.

Reviewed by:	tychon