Add dot to character set of candidate domain names

[wander]

We use [a-z0-9-] as character set for candidate names. Some zones may use multi-label names, i.e. names with a dot.

Priority is nice-to-have, as the big TLD players won't use multi-label names in their zone.

Note 1: do not put the dot as 0x2E character into the binary representation, replace each '.' from presentation format into binary label length (see nsec3.name_to_bin(...)).

Note 2: other characters may be used in weird zones, too. Everything from 0x00 to 0xFF is allowed in DNS.