Add a jitter buffer in the common USB serial driver code which
temporarily stores characters if the TTY buffer is full when
used a as a console. This can happen when a console is suspended.
Also properly do the flow stop signalling when this happens and
flow start when the condition changes back to normal again.

Bump __FreeBSD_version to force external kernel modules
to be recompiled. No kernel API changes.

MFC after: 1 week
Suggested by: ed @