Scan all iio contexts.

[teoperisanu]

Currently libm2k will detect only USB contexts. This restriction is found here: https://github.com/analogdevicesinc/libm2k/blob/master/src/contextbuilder.cpp#L76.

After libiio v.0.20 multiple backend types can be handled with the same function, using as parameter a concatenated string such as: "local:usb:ip:", or NULL for all the available backends.