wpa_supplicant: systemd service directives

Change the Requires= directive to BindsTo=.  This will cause
wpa_supplicant -i<iface> to terminate if <iface> disappears.

From 399ec170: Use correct (multi-user) target when installing systemd
units [...] all services should always be installed in multi-user.target

These directives are documented in systemd.unit(5)