dns/dnstable-convert: Upgrade to 0.12.0

* dnstable_convert: add -p option to not remove generated mtbl file
  with no records.

* dnstable_convert: additional logging at startup and exit.

* dnstable_convert: increase file descriptor resource limit at startup.

* dnstable_unconvert: sort rdata values in "wdns_sort_rrset" order
  for consistency with other dnstable data sources.

The problem fixed by patch-dnstable__convert.c has been fixed upstream.

Sponsored by:	DomainTools LLC