Make ownership and permissions for dnstap sockets configurable
Currently the unix sockets for dnstap capture are created with the same owner and group as dns-probe's process and mode 755. This is problematic when for example dns-probe is run as a systemd service under root user and a non-root source of dnstap data wants to connect to the sockets.
We could make ownership and permissions for dns-probe's dnstap sockets configurable with some sane defaults (probably same as the current state).