package/shairport-sync: fix init script (stop)

package/shairport-sync: fix init script (stop)

shairport-sync creates its pidfile at /var/run/shairport-sync/, so pass
that path to start-stop-daemon in the stop operation.

Also pass the executable path, allowing start-stop-daemon to check if
the PID matches the shairport-sync process, preventing killing some
other inocent daemon.

Fixes:
  https://bugs.busybox.net/show_bug.cgi?id=11566

Reported-by: Bin Zhang <yangtze31@gmail.com>
Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>