Document sort_io_queue sysctls/tunables.

MFC after:	1 week
filedesc assert that table size is at least 3 in fdsetugidsafety

Requested by: kib
Untangle iSCSI authentication code by splitting off the CHAP
implementation.

Reviewed by:	mav@
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation