dnstable_dump --rdata and --rrset switches don't work

[reedjc]

The dnstable_dump usage and manpage document the --rdata and --rrset switches.

[-r] or --rrset output rrset records
[-d] or --rdata output rdata records

Using the long argument fails like:
dnstable_dump: usage problem, '--rdata' might be 'rdata_names' or 'rdata'

It uses the argv library included in libmy. I checked the newer argv code. There is no option to do exact matches (or don't do partial matches). Also argv doesn't document the long arguments.
https://256stuff.com/sources/argv/argv.html#Long-Versus-Short

So when --rrset_names and --rdata_names switches were added, it broken long arguments for --rrset and --rdata.

As a workaround, the short arguments work.

When this is resolved, also add unit tests for the switches.