debian: Remove timer to setup repositories properly

When removing systemd services, dh_installsystemd has no idea about services
present in the older versions that been removed. So, it generates no handling
scripts for those services. No convenience methods exist too.

dh_installsystemd uses deb-systemd-helper which maintains state files in
/var/lib/systemd/deb-systemd-helper-enabled/*.dsh-also. These files need to
removed apart from symlinks created in /etc/systemd/system enable services are
enabled. If the service/timer is currently running, stop it too.

Use deb-systemd-helper and deb-systemd-invoke to achieve this.

Closes: #1835

Tests performed:

- Install a freedombox version older than 20.5. Upgrade to 20.5. Notice the
incorrect symlink
/etc/systemd/system/multi-user.target.wants/freedombox-setup-repositories.timer
and also the state file
/var/lib/systemd/deb-systemd-helper-enabled/freedombox-setup-repositories.timer.dsh-also
Then upgrade to a .deb packages built with the patch. The two files will be
removed. No extra messages are warnings are printed during package upgrade.

- Install a freedombox version older than 20.5. Upgrade to .deb packages built
with this patch. Notice the same results.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

debian/freedombox.preinst

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+set -e
+
+case "$1" in
+    upgrade)
+        # Handle removing freedombox-setup-repositories.timer from 20.5.
+        if dpkg --compare-versions "$2" le 20.7; then
+            if [ -x "/usr/bin/deb-systemd-invoke" ]; then
+               deb-systemd-invoke stop freedombox-setup-repositories.timer >/dev/null 2>/dev/null || true
+            fi
+
+            if [ -x "/usr/bin/deb-systemd-helper" ]; then
+                deb-systemd-helper purge freedombox-setup-repositories.timer >/dev/null || true
+                deb-systemd-helper unmask freedombox-setup-repositories.timer >/dev/null || true
+            fi
+
+            if [ -d /run/systemd/system ]; then
+                systemctl daemon-reload
+            fi
+        fi
+        ;;
+esac
+
+#DEBHELPER#
+
+exit 0