src/adns.h, src/types.c: Now adns_rr_addr can hold a sockaddr_in6.

This uses the recently introduced variable-size record structure
machinery so that only clients compiled against the new header file see
the larger record structure.

Note that we don't even try to actually do lookups for IPv6 addresses.
That will come later on.  This is just about sorting out the binary
interface.

Arrange that regress/case-brokenmail.sys asks for the correct record
size, because otherwise the answers get printed out wrongly.

[ I have done git-grep sizeof on a version of the head of this series
and found no un-corrected uses of sizeof(adns_rr_addr) -iwj. ]

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