util: Add generic frequency/channel command line handler

util: Add generic frequency/channel command line handler

The ability to parse channel definitions is required in a lot of places
inside iw. However, right now each of these duplicates a lot of code to
handle it.

So add a new helper which can be used everywhere.

Signed-off-by: Benjamin Berg <benjamin.berg@open-mesh.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>