fix: Correct MX value docs and validate admin record names

[ZPascal]

Summary

• README documented MX record values as a bare hostname, but the DNS spec (and this codebase's own probeRR validation via dns.NewRR) requires a preference before the exchange host, e.g. 10 mail.example.com.. Following the old docs made every MX creation fail with invalid_record_value. Docs are now corrected, and the previously untested MX create → serve path has test coverage (TestAdminCreateMXRecord, TestAnswerManagedMXRecord) proving the existing validation/serving logic was already correct.
• Hardened admin record name validation: adminCreateRecord/adminUpdateRecord previously only rejected an empty name, so a name containing whitespace or control characters (e.g. a newline) would be stored and later spliced unescaped into the RR string built for DNS responses. Added validRecordName (label length/charset checks, blocks whitespace/newlines, allows _-prefixed labels and wildcard *) and wired it into both handlers.

Test plan

• go build ./...
• go vet ./...
• go test ./...