gpsd: add hotplug handling

gpsd: add hotplug handling

The 'gpsd' offers the possibility to call the script '/etc/gpsd/device-hook'
when a GPS source was added or removed via gpsdctl.

In addition to the '/etc/gpsd/device-hook' call an event is now triggered
too after the 'gpsd' has started. This allows scripts to configurre 'gpsd'
receivers.

The following events are available for '/etc/hotplug.d/gpsd' scripts:

* ACTIVATE   via '/etc/gpsd/device-hook'
* DEACTIVATE via '/etc/gpsd/device-hook'
* STARTED    via '/etc/init.d/gpsd'

Signed-off-by: Florian Eckert <redacted>