Resync bring_interface_up with netctl.
Improve stop() function and make dhcpcd exit properly.

- do not use ip_unset and company from inside the inner layer,
  instead, rely on wpa_actiond's LOST/DISCONNECT events
- do not wait for wpa_actiond in the inner layer,
  instead, rely on systemd's kill facilities.
Use "systemctl stop --no-block" to stop netctl-auto-ng.

This avoids blocking on wpa_actiond which takes 5-7 seconds to kill
itself after signalling.