Support IPv6 in addr queries.

This is the end-game.  Add flags and options to allow clients and
administrators to choose which address families they want returned.  For
logical compatibility with old clients, don't return foreign address
families unless explicitly requested.

Lots of tests are modified to select only IPv4 addresses.  There's
nothing else up my sleeve.  A new test is added for the additional
functionality; more are expected later.

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