device: avoid signals sent to the mbim-proxy process

If e.g. mbim-proxy is started by ModemManager and we send a Ctrl+C to
it, the signal would be propagated to the mbim-proxy process and we
would kill it right away, while leaving ModemManager still around
wondering why the socket to the proxy got a HUP.

Avoid this, by making sure the mbim-proxy gets its own process group.
1 file changed