dvbsrc: smarten-up delsys autodetection logic

dvbsrc: smarten-up delsys autodetection logic

When there's no explicit delivery system information
for a channel in the channel configuration file and
the user hasn't selected one via setting the delsys
property, we *guessed* it by selecting the last
supported delsys reported by the driver. This change
provides the basis for smarter delsys auto detection
and implements a rule for DVB-T2. Rules for other
delivery systems can be added in _guess_delsys() in
a similar way.

Additionally: Store list of adapter-supported
delivery systems instead of querying the driver each
time this information is needed.

Related to:

https://bugzilla.gnome.org/show_bug.cgi?id=765731