Also adds the user, group and mode sub-args to -a, to set
permissions on the path created by -a for UDS.

Add the bogo_ip pseudo-VSA, representing IPv4 0.0.0.0:0, to be
exposed in VCL for non-IP addresses.

Also adding the field listen_sock to struct sess: pointer to the
struct listen_sock that was created by the acceptor and lives in
heritage.socks. This makes information like the endpoint name
(named -a arg) and the UDS path available from an sp.