Refactor Debian/Ubuntu package

Move files for packaging outside the docs directory into its own
packaging directory. Replace the existing postinstall and postremove
scripts with Debian maintainerscripts to behave more like a typical
Debian package:

* Start and enable the headscale systemd service by default
* Does not print informational messages
* No longer stop and disable the service on updates

This package also performs migrations for all changes done in previous
package versions on upgrade:

* Set login shell to /usr/sbin/nologin
* Set home directory to /var/lib/headscale
* Migrate to system UID/GID

The package is lintian-clean with a few exceptions that are documented
as excludes and it passes puipars (both tested on Debian 12).

The following scenarious were tested on Ubuntu 22.04, Ubuntu 24.04,
Debian 11, Debian 12:

* Install
* Install same version again
* Install -> Remove -> Install
* Install -> Purge -> Install
* Purge
* Update from 0.22.0
* Update from 0.26.0

See: #2278
See: #2133
Fixes: #2311

packaging/deb/prerm

@@ -0,0 +1,34 @@
+#!/bin/sh
+# prerm script for headscale.
+
+set -e
+
+# Summary of how this script can be called:
+#        * <prerm> 'remove'
+#        * <old-prerm> 'upgrade' <new-version>
+#        * <new-prerm> 'failed-upgrade' <old-version>
+#        * <conflictor's-prerm> 'remove' 'in-favour' <package> <new-version>
+#        * <deconfigured's-prerm> 'deconfigure' 'in-favour'
+#          <package-being-installed> <version> 'removing'
+#          <conflicting-package> <version>
+# for details, see https://www.debian.org/doc/debian-policy/ or
+# the debian-policy package.
+
+
+case "$1" in
+    remove)
+      if [ -d /run/systemd/system ]; then
+          deb-systemd-invoke stop headscale.service >/dev/null || true
+      fi
+    ;;
+    upgrade|deconfigure)
+    ;;
+
+    failed-upgrade)
+    ;;
+
+    *)
+        echo "prerm called with unknown argument '$1'" >&2
+        exit 1
+    ;;
+esac