syncthing: Use systemd-sysusers for creating a system user account

- Drop dependency on 'adduser' package.

Tests:

- Functional tests for syncthing work.

- Installing syncthing app works. The system user and group are created with
proper UID/GID, shell, gecos, and home directory.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/syncthing/data/usr/lib/sysusers.d/freedombox-syncthing.conf

@@ -0,0 +1,6 @@
+# Create system user and group to run syncthing as system daemon (as opposed to
+# running in particular user's account). The user is created at boot or when
+# 'systemd-sysusers freedombox-syncthing.conf' is run. To override this file as
+# system administrator, create a file /etc/sysusers.d/freedombox-syncthing.conf
+# or link it to /dev/null.
+u! syncthing - "Syncthing file synchronization server" /var/lib/syncthing

plinth/modules/syncthing/privileged.py

@@ -1,9 +1,7 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 """Configure Syncthing."""
 
-import grp
 import os
-import pwd
 import shutil
 import time
 
@@ -32,21 +30,8 @@ def augeas_load(conf_file):
 @privileged
 def setup():
     """Perform post-install actions for Syncthing."""
-    # Create syncthing group if needed.
-    try:
-        grp.getgrnam('syncthing')
-    except KeyError:
-        action_utils.run(['addgroup', '--system', 'syncthing'], check=True)
-
-    # Create syncthing user if needed.
-    try:
-        pwd.getpwnam('syncthing')
-    except KeyError:
-        action_utils.run([
-            'adduser', '--system', '--ingroup', 'syncthing', '--home',
-            DATA_DIR, '--gecos', 'Syncthing file synchronization server',
-            'syncthing'
-        ], check=True)
+    # Create a 'syncthing' system user and group, if needed.
+    action_utils.run(['systemd-sysusers', 'freedombox-syncthing.conf'])
 
     if not os.path.exists(DATA_DIR):
         os.makedirs(DATA_DIR, mode=0o750)