TV: Automatically select a free card when changing channel

[Lawrence Rust <lvr@…>]

This patch fixes a shortcoming in TV::ChangeChannel? which presently doesn't detect if the new channel is unavailable on the current card due to a current recording or other LiveTV access. When the channel change request is sent to the backend it detects the conflict and selects a different channel to view (on the requested card) even when there are cards available that could tune the requested channel. This patch checks the current card for conflicts and if required, switches to the next available card that can tune the channel. It has most utility in systems with multiple tuners and in installations where there are multiple frontends,

[sphery]

After the recent fixes to Live TV channel change (and along with the additional user control provided by the recently-added Live TV/recording order), this should no longer be necessary.