[PR-206] dvb: extend the multi-frontend PR to include cleaner support.

Generally this is intended to be used such that a user can select which
frontend they wish to enable. However in theory with clever configuration,
configure one adapter then the other and enabling close fds and disabling
idle scan, it might be possible to use both frontends.

But since TVH doesn't do any form of arbitration strange errors might
occur if you try to access both frontends at the same time (which for
most such adapters is not possible).