Reentrancy: Introduce adns__returning

Replace all the adns__consistency calls on the return paths from adns
with adns__returning (except in setup.c, whose functions never set up
any queries).

I have separately gone through the public entrypoints in adns.h and
checked that they have the expected adns__consistency calls.  So with
this patch, the new imprecation about adns__returning is satisfied.

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