wpa_supplicant service: restart instead of stop & start

We now wait for dhcpcd to acquire a lease but dhcpcd is restarted on
system activation. As wpa_supplicant is stopped while dhcpcd is
restarting a significant delay is introduced on systems with wireless
network connections only. This changes the wpa_supplicant service to
also be restarted together with dhcpcd in case both services were
changed.