MINOR: stream-int: add new functions si_{rx,tx}_{blocked,endp_ready}()

MINOR: stream-int: add new functions si_{rx,tx}_{blocked,endp_ready}()

The first ones are used to figure if a direction is blocked on the
stream interface for anything but the end point. The second ones are
used to detect if the end point is ready to receive/transmit. They
should be used instead of directly fiddling with the existing bits.