Add a configuration knob for record validation strictness

[Kris Deugau]

Consider adding a configuration knob for record validation.

Three basic modes:

• "strict": pre- ~ r606 behaviour; only logically useful types are allowed in the relevant zones (ie, A, CNAME, MX, and SRV records are only allowed in forward zones, PTR records are only allowed in reverse zones)
• "relaxed": behaviour completed as of ~ r630; all real record types are allowed in all live zones, and in default records. Note that some combinations are allowed through as "valid" because there is no sane way to actually test them for any kind of correctness. IP values are still normalized and converted to or from .arpa names in many cases.
• "open": Validate for form and pray the user has a clue.