Import/tinydns: when importing ip6.arpa records,

provide a 64-bit zone hint to get_zone_id. It will often work.
Import/Base: when finding a zone, search for left-

most domain label and work all the way to the right (previously, only tried 3x, which doesn't work at all for IPv6 rDNS zones)
Import/tinydns: define MX pref to 0 if undef
Zone/Record: permit _ in host names

as well as any other ASCII printable char in names