webcamd: new rc.conf variable. Find USB devices by name.

Changes:

* Improved the comments at the top. For clearer examples and descriptions of variables.
* New variable to detect any attached USB devices simply by their USB device name. This is the new recommended way.
* Improved an existing mis-configuration error message.
* New rc.d start message to report that webcamd has either started up correctly. Or that webcamd has failed to start.