Introduce fall-back using remote enigma2 receiver.

First shot at a feature to enable using another enigma2 receiver
as fall-back. The idea is that enigma2 uses a tuner from another
receiver automatically, whenever a service cannot be selected
(view/record/stream/pip) because no tuner is available (either
because it is busy with another service or because there are no
tuners that can handle this service (dvb-s/dvb-c/dvb-t) etc.)

At least for the moment this has one major drawback: when a
service cannot be selected locally, the remote service is always
started, even if the remote receiver cannot handle the service.
Unfortunately there is, afaik, no mechanism to probe a remote
enigma2 instance whether a service "is playable" like there is for
local services.

If no "remote fallback tuner" is configured (default), service
selection works as usual and the above drawback does not apply.
Merge branch 'master' of git://git.code.sf.net/p/openpli/enigma2 into 4.2-development