udp: Insist on only one successful default socket setup
If we default the addresses to use, we only insist that at least one
of them can be successfully set up.
(This is particularly significant when upgrading an existing secnet
installation to the new secnet, with IPv6 support, but on a host with
no IPv6, and no address configured in the config file. Without this
patch, secnet would fail to start up because it would try, and fail,
to bind the IPv6 address.)
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
~mdw / secnet / commit