Streamline configuration and operation of dns forwarding service in
following ways:

• Rename dns.py to dns_forwarding.py to better align with the operation mode path for the service
• Templatize systemd override for pdns-recursor.service and move the generated override files in /run. This ensures that the override files are always generated afresh after boot
• Simplify the systemd override file by removing the redundant overrides
• Relocate configuration path for pdns-recursor to /run/pdns-recursor and utilize the RuntimeDirectory default that pdns-recursor expects
• We do not need to use custom --socket-dir path anymore, the default path (viz., /run/pdns-recursor is fine)