Proposal: stricter domain hint requirements

Background
---
Right now a log is expected to look up a submitter's public key hash via DNS.  A
domain hint, say, example.com, specifies the location of a TXT RR that contains
the appropriate key hash in hex-encoding.  "Some domain knows about the key".

Downsides with this:
1. A log can be instructed to look up arbitrary TXT records
2. No versioning

As far as we know there are no amplification threats with (1), but ideally it
would only be possible to query TXT RRs that are actually relevant for Sigsum.

Not having any versioning could potentially become a headache.  All other log
endpoints are versioned.  There is no good reason to not have versioning here,
unless that would imply something like registering many different things with
IANA as a result.

Proposal
---
Require that a domain hint is formatted as:

	_sigsum_v0.*

Examples of valid domain hints:

	_sigsum_v0.com
	_sigsum_v0.example.com
	_sigsum_v0.sub.example.com

Examples of invalid domain hints:

	_sigsum_v0hello.example.com

This change addresses both (1) and (2), without making DNS configs harder.

Notes
---
For v1 we need to consider if something should be registered with IANA.  Credit
to Patrik Wallström who pointed us towards documentation about labels with
underscores:
	* https://www.rfc-editor.org/rfc/rfc8552.html
	* https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#underscored-globally-scoped-dns-node-names

Note also that the dependency on TXT look-ups means that a "hidden log" via Tor
would need help from a resolver that is also available over Tor (preferably an
onion but at minimum reachable over TCP).  This is because TXT records cannot be
resolved over Tor.  This proposal allows the used resolver to be restricted to
only resolve _sigsum_*.