[NMC] support domain references and mappings

[taoeffect]

Handle "map", and magical references such as "", "." (?), "@" etc. See:

• https://wiki.namecoin.info/index.php?title=Domain_Name_Specification#Domain_references
• https://wiki.namecoin.info/index.php?title=Domain_Name_Specification#Sub-domains

Consider that some sites still use legacy mapping format like {"map":{"":"74.207.231.13"}} (d/bluishcoder for example), but this is discouraged and will be obsoleted according to:

• https://wiki.namecoin.info/index.php?title=Domain_Name_Specification#Legacy_support

Decide whether or not to support this legacy stuff (and if so, create another issue for that separately).

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[pysiak]

In dnschain debug logging I usually see lookups of RESERVED domains or legacy ones. The namecoin wiki says this features is still supported but will be obsoleted at some point. Perhaps if it's supported from the specification's perspective then it makes sense to still support it in the implementation.

Unless we know when this is going to be obsoleted, and it's a reasonable timeframe to start holding the banner of change now and maybe return FORMERR for this as a hint that namecoin dictionary value is not conforming to the format.

If not, I'd vote for supporting it.