Only allow selection of channels that can be enabled

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

This function will create a network context if the environment variable
OSC_REMOTE is set (to the host's name or IP), or a local context if it
is not set.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
DMM plugin: update to use osc_create_context()

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
iio_widget.c: Updated to use libiio

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
plugins/AD5628_1.c: Updated to use new iio_widget.c / libiio

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
plugins/AD7303.c: Updated to use new iio_widget.c / libiio

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
plugins/fmcomms2.c: Updated to use new iio_widget / libiio

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
plugins/fmcomms1.c: Updated to use new iio_widget / libiio

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
plugins/motor_control.c: Updated to use new iio_widget / libiio

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