Make private function iio_device_process_samples visible

This function is to be used from within IIOD, so it has to correspond
to a public symbol in the library, even if it is not part of the API.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
IIOD: Exit reading thread if iio_device_read_raw fails

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
IIOD: Use private API

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
IIOD: Added the -D option to enable server-side demuxing of channels

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>