add support for multiple copies of the same transport on different frequencies

[Karl Egly]

When receiving DVB-T(2) transmissions from multiple networks/transmitters it is possible to receive the exact same transport (same original_network_id and transport_id) on multiple frequencies. As currently the transport_id is the primary key of the transport we can not handle copies and end up with "last scanned frequency wins". The DVB specs mandate that we import all transports and channels, then collapse multiple instances of the same and choose a sane default (e.g. strongest signal first).

[paulh]

See also #12360