`--control` as the last argument read `args.next()` as None and reset the
port taken from CADCRAFT_CONTROL_PORT, without logging anything, so
`CADCRAFT_CONTROL_PORT=47321 cadcraft --control` started no control
channel. A bare `--control` now keeps the variable's port and logs a
warning either way; a port given on the command line still wins. The
precedence is documented in docs/control-protocol.md.
Marco Arnulfo
3f709bed2e