unloading a driver that Click depends on

[Eddie Kohler]

Current CVS source allows you to unload a device driver while Click is
active. The relevant device elements will just stop doing anything. If a
matching device is later brought up, the elements will seamlessly start
using that device.

There also is a new keyword argument to these elements, ALLOW_NONEXISTENT.
If ALLOW_NONEXISTENT is true, then Click will allow the configuration to
initialize even if an element refers to a nonexistent device. Again, if a
device is plugged in later that matches, the element will use that device.

Benjie, would you mind doing a bit of testing with this for PollDevice? I
don't have a polling device handy. Like 2 minutes.

Thanks,
love,
ed