installer: use a drop-in to disable systemd-resolved stub listener

systemd-resolved supports drop-in config snippets, e.g. by placing .conf
files in /etc/systemd/resolved.conf.d. During install, use a drop-in
config to set DNSStubListener=no, instead of modifying the main config.

This is generally better practice, and also prevents conflicts when
distribution packages are upgraded, which may include new versions of
/etc/systemd/resolved.conf.

Signed-off-by: Nick Rosbrook <enr0n@ubuntu.com>

automated install/uninstall.sh

@@ -94,8 +94,9 @@ removePiholeFiles() {
     echo -e "  ${TICK} Removed config files"
 
     # Restore Resolved
-    if [[ -e /etc/systemd/resolved.conf.orig ]]; then
-        ${SUDO} cp -p /etc/systemd/resolved.conf.orig /etc/systemd/resolved.conf
+    if [[ -e /etc/systemd/resolved.conf.orig ]] || [[ -e /etc/systemd/resolved.conf.d/90-pi-hole-disable-stub-listener.conf ]]; then
+        ${SUDO} cp -p /etc/systemd/resolved.conf.orig /etc/systemd/resolved.conf &> /dev/null || true
+        ${SUDO} rm -f /etc/systemd/resolved.conf.d/90-pi-hole-disable-stub-listener.conf
         systemctl reload-or-restart systemd-resolved
     fi