Squashed commit of the following:

    drivers/usbdev/cdcacm.c:  Change design for queuing RX packets that cannot be processed.  Previous design had a logic problem that could cause data loss.

    drivers/usbdev/cdcacm:  Fixes one of two know design issues.

    drivers/usbdev/cdcacm:  First attempt to plug data leak in input flow control design.  Still missing a few things.