Add open-p slot to FUNDAMENTAL-STREAM and update default methods

[yitzchak]

Currently the CLOSE and OPEN-STREAM-P implementations do not conform to the Gray stream proposal. Specifically, the Gray stream proposal states the following for CLOSE and OPEN-STREAM-P:

  CLOSE  stream &key abort			[Generic Function]

    The existing function CLOSE is redefined to be a generic function, but
    otherwise behaves the same.  The default method provided by class
    FUNDAMENTAL-STREAM sets a flag for OPEN-STREAM-P.  The value returned
    by CLOSE will be as specified by the issue CLOSED-STREAM-OPERATIONS.

  OPEN-STREAM-P stream				[Generic Function]

    This function [from proposal STREAM-ACCESS] is made generic.  A
    default method is provided by class FUNDAMENTAL-STREAM which returns
    true if CLOSE has not been called on the stream.

I've added a slot OPEN-P to FUNDAMENTAL-STREAM and added the needed default methods.

Also fixed the STREAM-LISTEN method so that it returns NIL when STREAM-READ-CHAR-NO-HANG returns :EOF.