cam(4): Add an xpt-neutral flag indicating a valid panic CCB

No functional change.

Note that this change is careful to set the CCB header xflags after
foo_fill_bar() routines, which generally zero existing flags. An earlier
version of this patch mistakenly set the flag before the fill routines.

Submitted by: Scott Ferris <sferris AT isilon.com>, jhibbits@
Reviewed by: bdrewery@, markj@, and non-committer FreeBSD contributor Anton Rang
Sponsored by: Dell EMC Isilon