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

[adns] / regress / case-noinfto.out

adns debug: using nameserver 172.18.45.36
a.b.c.d flags 0 type 1 A(-) submitted
a.b.c.d flags 0 type A(-): DNS query timed out; nrrs=0; cname=$; owner=$; ttl=604744
rc=0