added -pfextra CLI flag to nt_export invocation

it's always performed in tinydns, flag required for running 'make' targets after export with BIND
BIND export: if mailaddr blank, insert a default
tinydns export: replace AAAA export method

replaced the AAAA method imported from djbdnsRecordBuilder.
added DNSKEY export support for BIND & tinydns
added DS record export support