INT 2F U - Network - RELINK KEYBOARD HANDLER
        AX = B808h
        ES:BX -> INT 09 handler network should call after it finishes INT 09
Notes:  this call replaces the address to which the network software 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 the network gets the INT 09 first unless the new handler
          completely takes over INT 09 and never chains.
        called by DOS 3.2 KEYBxx.COM