INT 2F - PCIBM 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.
SeeAlso: AX=B808h