argparse is error-prone in that `action='append'` starts from (a copy of) the
given default when it adds values from command-line, rather than restarting
from an empty list. For example, simply passing `--disable-proto udp` resulted
in ['udp', 'udp6', 'udp'], which caused 'udp6' to remain disabled.

/reviewed-on nexedi/re6stnet!17