software/powerdns: use shorter path for socket

powerdns uses a socket named pdns.controlsocket in socket-dir (
https://doc.powerdns.com/authoritative/running.html#control-socket )
We sometimes see errors on test nodes that this path is too long:

     Aug 13 20:54:10 Unable to open controlsocket, path '/srv/slapgrid/slappart13/srv/testnode/cvr/inst/test0-0/tmp/inst/pdns1/var/run/pdns-socket/pdns.controlsocket' is not a valid UNIX socket path.

Instead of using "var/run/pdns-socket", put the socket in "var/run",
which should be short enough.