src/types.c: Start on IPv6 support in adns_r_addr queries.

src/types.c: Start on IPv6 support in adns_r_addr queries.

The way addr queries work internally is now very different.  The
toplevel query is now `virtual', and exists to spawn subqueries for each
applicable address type (currently hardcoded to A only, but this will
change later).

The toplevel query has a callback function which assembles its answer
from the various child queries, and there's a collection of support
machinery too.  (Some of the bookkeeping done during this may seem
unnecessary, but it will come in handy later.)

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