Allow arbitrary namecoin keys

[taoeffect]

Remove VALID_NMC_DOMAINS restriction on namecoin keys.

[taoeffect]

But, for d/ add the following two checks as per request from @ryancdotorg:

• Do not allow start with: d/xn-- to protect against homoglyph attacks (although browsers mitigate already...). I'll wait for them to reach consensus on this and update the wiki.
• Filter with ^[a-z]([a-z0-9-]{0,62}[a-z0-9])?$ from their spec