Merge from hpt27xx, r249468 (mav):

MFprojects/camlock r248982:
Stop abusing xpt_periph in random plases that really have no periph related
to CCB, for example, bus scanning.  NULL value is fine in such cases and it
is correctly logged in debug messages as "noperiph".  If at some point we
need some real XPT periphs (alike to pmpX now), quite likely they will be
per-bus, and not a single global instance as xpt_periph now.
Document that a literal jail name of 0 (zero) is not allowed.

PR:		174436
Submitted by:	Robert Schulze
Reviewed by:	mjg
MFC after:	3 days