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.

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