Add a wpa_supplicant hook script.

Add a wpa_supplicant hook script.

This script starts wpa_supplicant on a wireless interface if not
already running and /etc/wpa_supplicant.conf exists, stops it
when the interface departs and sends wpa_supplicant a reconfigure
command.

If dhcpcd is running in master mode, you can use `dhcpcd -n eth0'
it instruct wpa_supplicant to reload its configuration file.