additional refactoring of dns_validator

refactor validator so that the validation status object (previously called dns_valstatus_t, which was derived from dns_validatorevent_t), is now part of the dns_validator object. when calling validator callbacks, the validator itself is now sent as the argument.

(note: this necessitates caution in the callback functions that are internal to validator.c validators spawn other validators, and it can be confusing at times whether we need to be looking at val, val->subvalidator, or val->parent.)