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.)

Because more of the work is now done in internal query callbacks, some
of the answers in the tests are reported in different order.  See
`Reentrancy: Avoid reentrant callbacks' for more detailed explanation
of a similar change.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>