src/: Support IPv6 reverse queries.

This involves moving most of the label parser in ckl_ptr, and the
reverse-domain constructor, into addrfam and teaching them about IPv6.

The label parser is a little tricky because it involves keeping track of
a parse state for each possible address family until we run out of
possibilities or the parse is complete.

Moving the reverse-domain constructor introduces a slightly unpleasant
hack: because knowledge of the right reverse-lookup zone belongs in
addrfam, we get adns_submit_reverse to pass a sentinel through
_reverse_any to addrfam.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>