src/setup.c, src/transmit.c: New function finds udpsocket by AF.

[adns] / regress / case-polltimeout.out

adns debug: using nameserver 172.18.45.36
chiark.greenend.org.uk flags 0 type 1 A(-) submitted
chiark.greenend.org.uk flags 0 type A(-): DNS query timed out; nrrs=0; cname=$; owner=$; ttl=604770
rc=0