Run the long-lived process as the configured non-root user (#31)

* Run the long-lived process as the configured non-root user

start.py used to stay root for its whole life; only the individual
seaf-cli/seaf-daemon subprocess calls were unprivileged, via a per-call
runuser wrapper. The orchestrating Python process itself, including the
RPC socket to seaf-daemon and the watch_status() loop, ran as root the
entire time.

main() now drops privileges in-process (dsc.misc.drop_privileges, using
os.initgroups/setgid/setuid) right after setup_uid()/create_dir() -
which still need root to remap the seafile user's UID/GID and chown
/dsc - and before SeafileClient is constructed. Since the process is
already the target user by the time any seaf-cli call happens, runuser
is removed from user_cmd(); it also could not be invoked from an
already non-root process.

Verified against the real image, not just mocked unit tests: a running
container's PID 1 shows Uid: 1000 1000 1000 1000 in /proc/1/status, and
tests/smoke.sh gained a check exercising setup_uid/create_dir/
drop_privileges together against the image's real /etc/passwd.

* Isolate os.environ mutation in drop_privileges test

drop_privileges() writes HOME/USER/LOGNAME directly to os.environ, and
the test asserting that didn't restore them, leaking state into later
tests. monkeypatch.delenv registers the keys so its teardown restores
them regardless of who wrote to os.environ.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>

start.py

@@ -9,7 +9,7 @@ import sys
 from dsc import SeafileClient, const
 from dsc.config import env_bool, env_int, env_str
 from dsc.errors import ConfigError, DscError, GracefulShutdown
-from dsc.misc import setup_uid, create_dir
+from dsc.misc import setup_uid, create_dir, drop_privileges
 from dsc.paths import plan_lib_dirs
 from dsc.secrets import resolve_secret
 
@@ -148,6 +148,7 @@ def main():
 
     setup_uid(args.uid, args.gid)
     create_dir(const.DEFAULT_APP_DIR)
+    drop_privileges(args.uid, args.gid)
 
     client = SeafileClient(args.server, args.username, password,
                            const.DEFAULT_APP_DIR, token=token)