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

- Drop dependency on 'adduser' package.

Tests:

- Functional tests for infinoted work.

- Installing infinoted 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/infinoted/data/usr/lib/sysusers.d/freedombox-infinoted.conf

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

plinth/modules/infinoted/privileged.py

@@ -1,10 +1,8 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 """Configure infinoted."""
 
-import grp
 import os
 import pathlib
-import pwd
 import shutil
 import subprocess
 import time
@@ -126,21 +124,8 @@ def setup():
 
     action_utils.service_daemon_reload()
 
-    # Create infinoted group if needed.
-    try:
-        grp.getgrnam('infinoted')
-    except KeyError:
-        action_utils.run(['addgroup', '--system', 'infinoted'], check=True)
-
-    # Create infinoted user if needed.
-    try:
-        pwd.getpwnam('infinoted')
-    except KeyError:
-        action_utils.run([
-            'adduser', '--system', '--ingroup', 'infinoted', '--home',
-            DATA_DIR, '--gecos', 'Infinoted collaborative editing server',
-            'infinoted'
-        ], check=True)
+    # Create an 'infinoted' system user and group, if needed.
+    action_utils.run(['systemd-sysusers', 'freedombox-infinoted.conf'])
 
     for directory in (DATA_DIR, KEY_DIR, SYNC_DIR):
         if not os.path.exists(directory):