src/types.c: Support multiple address families in adns_rr_addr lookups.

src/types.c: Support multiple address families in adns_rr_addr lookups.

This is now largely working.  A query for adns_r_addr now causes (via a
new type hook) concurrent child queries for all of the wanted address
families; we then stitch the answers together and report the result.

Astonishingly, we have NS, MX, and SRV queries doing approximately the
right thing.

Significant pieces missing:

  * init flags and configuration aren't yet processed;

  * (pap_hostaddr) we should query for wanted rrtypes but which aren't
    in the additional section, even if we got some other rrtypes for the
    name;

  * CNAME records aren't handled at all.