Add 'xfr' subcommand for AXFR and IXFR.

[Philip-NLnetLabs]

Simple AXFR and IXFR client. Quite a bit of code duplication. Is it worth making that more general?

[ximon18]

Initial feedback:

• cargo update was needed before it compiled.
• I find the help text Usage: dnsi xfr <QUERY_NAME_OR_ADDR> confusing: what am I supposed to supply for QUERY_NAME_OR_ADDR? I see no mention of host, port, AXFR or IXFR or serial number?

[ximon18]

Ah, my bad, now I see there is more help text that I didn’t see somehow the first time.

Ahha, I know why. I had not used —help but just dnsi xfr with no other arguments. Then it unhelpfully displays just that one line of usage. This is not an issue with this PR but a more general issue with dnsi I think.

[ximon18]

Unfortunately I am unable to use the xfr subcommand in my current use case as I don't see any way to supply a TSIG key.

Additionally, if I supply --ixfr serial but use AXFR instead of IXFR there is no warning or error as I would expect.