Support OBEX FTP over USB

[James Henstridge]

Some people seem to use OBEX FTP over USB. If it isn't difficult, it'd be nice to support it.

Things that need to be addressed include:

 * how do you discover USB devices that can do this?
 * how do you establish a connection to such devices?

[James Henstridge (jamesh)]

Had another go at using OBEX over USB with my phone -- it looks like it was a simple permissions error stopping it from working previously, so I do have hardware that can test this.

It looks like the openobex interface for setting up USB connections is quite primitive though:

 * The API only gives you a list of the discovered connections, then lets you pick one to connect to.
 * For my phone, I get three possible OBEX endpoints discovered (all of which appear to act the same).
 * It doesn't seem to provide any way of mapping back from the discovered interface to the USB device.

So this makes it tricky to do useful things like asking HAL to notify us as interesting USB devices come and go, since we can't then use that data to establish a connection.