AP_HAL_PX4: use rc status flags and channel count for a more robust rc l...

...oss/failsafe handling

This change evaluates the rc status flags recently added to PX4Firmware to make the failsafe handling more robust. RC channel values are double-buffered now and forwarded only if there is no condition that could lead to zero channel values. Additonally, any rcin notifications with channel_count=zero are rejected.

Please note: these patches rely on the bug fixes made by https://github.com/PX4/Firmware/pull/798, please do not merge before cherry-picking these fixes.