Authenticate from mounted secrets and stop passing the password to seaf-cli

The password could only be given through the PASSWORD variable, so Docker
Compose and Swarm secrets were unusable and the password sat in the container
environment.

PASSWORD_FILE and TOKEN_FILE read a secret from a mounted file. Only trailing
newlines are stripped, since secret files are usually written with one and a
password that silently carries it just fails to authenticate; everything else,
including surrounding spaces, belongs to the secret. Setting both a variable
and a file for one secret is rejected rather than resolved by precedence,
because the two are meant to be the same value and quietly picking one hides
the mistake until authentication fails.

The installed seaf-cli supports token authentication and ignores the password
when a token is given, so sync now passes -T instead of -p. The account
password no longer appears in any command line. The token does, because
seaf-cli accepts credentials only as arguments, but a token can be revoked on
the server. This is documented in the README.

Both secrets are masked before a command is logged.

Tests cover newline handling, empty and missing files, conflicting sources,
and that neither secret reaches the logs. Requests are now blocked in tests:
the client retries with backoff, so a test that reached the network appeared
to hang instead of failing.

tests/test_daemon.py

@@ -25,7 +25,9 @@ def fast_polling(monkeypatch):
 @pytest.fixture
 def client(monkeypatch):
     monkeypatch.setattr(client_module, "create_dir", lambda path: None)
-    return SeafileClient("seafile.example", "user", "pw", "/dsc")
+    # A token keeps the tests off the network: without one, authentication
+    # would be attempted against the server before syncing.
+    return SeafileClient("seafile.example", "user", "pw", "/dsc", token="tok-en")
 
 
 def fake_runner(returncodes, monkeypatch, stdout=b""):