Added API function iio_device_find_debug_attr

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Network backend: verify that attributes exist before reading/writing

This is done to avoid un-necessary commands.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Make iio_channel_attr_write_[bool|double|longlong] return 0 on success

As that's what is written in the documentation anyway.

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