Check for existing DNS server during installation

During installation, check for an existing DNS or HTTP daemon and then determine the best way to proceed with the install.

Some distro's have another webserver running, or possibly a master daemon like connmand that have their own DNS resolvers. Check and warn that this will not work very well.

Referenced from Ubuntu - No DNS queries · Issue #1089 · pi-hole/pi-hole · GitHub

Renamed the title so these this feature request and this one can be worked out in tandem.

Also making unlisted, but can be accessed from here: