Routine Description:

    This procedure raises IRQL to high_level, seizes the Debug port
    spinlock, and checks to see if a breakin packet is pending.
    If a packet is present, return TRUE, else FALSE.

    A packet is present if:

    There is a valid character which matches BREAK_CHAR.

    N.B.    Interrupts must be OFF around this call

Return Value:

    TRUE if breakin sequence present, caller should execute int-3.
    FALSE if no breakin seen.