Add a new and somewhat obscurely named sysctl,
security.cheri.debugger_on_sandbox_exception,  which will let you drop into
DDB when an in-sandbox exception occurs, making it easy to inspect the CHERI
trap frame (show cheriframe) and trusted stack (show cheristack).  Right now
we don't print out a frame on the console, but it would be useful to do that
as well.