Read the environment with typed parsers and apply the documented defaults

set_defaults() assigned os.getenv() results directly, which overwrote the
argparse defaults with None whenever a variable was unset. A default container
therefore ran "seaf-cli config -k delete_confirm_threshold -v None", and the
same for upload_limit and download_limit: none of the three documented
defaults were ever applied.

SEAFILE_UID and SEAFILE_GID had a second problem. argparse type=int applies
only to command line flags, never to set_defaults, so both stayed strings.
The check in setup_uid compares them against pwd integers, and a string is
never equal to one, so usermod ran on every start.

dsc/config.py reads each variable with its documented default and rejects a
malformed or negative value at startup, naming the variable, instead of
passing it on. An empty variable counts as unset.

Booleans are now rendered as true and false rather than Python's True and
False. The daemon reads disable_verify_certificate with its boolean accessor,
which does not recognise the capitalised form. I could not run the daemon to
confirm the previous value's effect, since the AppImage is x86_64 only.

seaf-cli is no longer given a None value for any option.

dsc/misc.py

@@ -65,3 +65,15 @@ def hide_secrets(cmd: list, *secrets: str) -> list:
             if secret in arg:
                 cmd[i] = arg.replace(secret, "********")
     return cmd
+
+
+def config_value(value) -> str:
+    """
+    Render a setting the way seaf-cli stores it.
+
+    Booleans are written as "true" and "false": the daemon reads this setting
+    with its boolean accessor, and Python's str() would produce "True".
+    """
+    if isinstance(value, bool):
+        return "true" if value else "false"
+    return str(value)