ALL: Make is_input_device callable by plugins

Signed-off-by: Robin Getz <robin.getz@analog.com>
ALL: Move element sorting/inserting functions into iio_utils,
     so they can be used by all plugins.

Signed-off-by: Robin Getz <robin.getz@analog.com>
DMM: Add new plugin ("DMM"), so we can look at non-streaming/no buffer
     ADCs. Rev 1 - still needs some work, but it's OK for testing

Signed-off-by : Robin Getz <robin.getz@analog.com>