[FIX] config: a list is always equal to itself

add_option(action=append*) always modifies the ``default`` list
in-place. When using DEFAULT_LOG_HANDLER directly, that means
log_handler is always equal to DEFAULT_LOG_HANDLER since they're the
same list object. Thus the --log-handler command-line would never
overwrite the log_handler value from the configuration file, which is
unexpected

Fix that by copying DEFAULT_LOG_HANDLER before passing it as the
option's default value.
[FIX] config: correctly serialize log_handler to CSV