1.0.5.55: interrupt safe REFILL-BUFFER/FD

1.0.5.55: interrupt safe REFILL-BUFFER/FD

 * Check for blocking / wait for new input before touching the stream.

 * Check that the SAP is still there after the wait, in case an interrupt
   handler or an FD handler closed the stream from under us.

 * Wrap the main action in WITHOUT-INTERRUPTS to prevent asynch unwinds
   from leaving the stream in an inconsistent state. (Since the read
   is going to be non-blocking it should be over soon enough.)

 * Arrange to signal errors outside the WITHOUT-INTERRUPTS.