Add assertion to catch duplicated notes.

Sponsored by:	The FreeBSD Foundation
Mark iscontrol(8) and iscsi_initiator(4) obsolete.

Differential Revision:	https://reviews.freebsd.org/D931
Reviewed by:	wblock@
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Mark CTL frontend's CAM driver as CAM_PERIPH_DRV_EARLY.

Target mode operation does not depend on the initiator mode scan process.
This change allows the target driver to attach earlier and receive some
async events (like AC_CONTRACT) that could be lost otherwise.

MFC after:	1 week
Update isp_tgt_map and send new arrival notification if target that departed
earlier has returned.  Previously that code worked only once, confusing CTL.

MFC after:	1 month
Fix r272936 build with old GCC.

MFC after:	1 week