INT 2F - PC Network RECEIVER.COM - UNLINK KEYBOARD HANDLER
        AX = B908h
        ES:BX -> INT 09 handler RECEIVER should call after it finishes INT 09
Note:   this call replaces the address to which RECEIVER.COM chains on an
          INT 09 without preserving the original value.  This allows a prior
          handler to unlink, but does not allow a new handler to be added
          such that RECEIVER gets the INT 09 first.