Add code for explicit selection of pooled A records.

To better cope with round robin pooled A records like keys.gnupg.net
we need to keep some information on unresponsive hosts etc. What we
do now is to resolve the hostnames, remember them and select a random
one. If a host is dead it will be marked and a different one
selected. This is intended to solve the problem of long timeouts due
to unresponsive hosts.

The code is not yet finished but selection works.