This is the second part of what used to be as66c.  It fixes the
initialization of the debugging macros in core/message.c and adds routines
to enable/disable endpoints, interfaces, and entire devices.  The code
that _uses_ these routines will come in the next patch.

 # Initialize the debugging macros properly in message.c.
 #
 # Add usb_disable_endpoint(), usb_disable_interface(), usb_disable_device(),
 # usb_enable_endpoint(), and usb_enable_interface().