Fast fail a validator deadlock

Fast fail a validator deadlock

We return DNS_R_NOVALIDSIG if we detected a deadlock. Then in
'validate_async_done()', this result value is used to check if we
need to fall back to insecure. As part of that we create a new fetch
but that fails because of the detected deadlock. This results in a loop
of deadlock detected, fallback to insecure, deadlock detected, ...

Add a new result value, ISC_R_DEADLOCK, and return this instead when
we have detected a deadlock. This will be treated as a generic error,
as there is no special handling for this result value.

(cherry picked from commit bc1d177cc2e82f18a77985c518e169f0e37d88c7)