nodewatcher: Change mechanism for client device detection

This is simpler than the previous approach and does not rely
on parsing.

This fixes:
- Interfaces being accounted for multiple times for certain
  devices
- Errors when output of bridge function changes (as with the
  current OpenWrt master)

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Fabian Bläse <fabian@blaese.de>