INT 2F - PTS-DOS 6.51, S/DOS 1.0+ - BACKDOOR INTERCEPT INT 2Fh CHAIN

	AX = 12F0h
	CX:DX -> user INT 2F handler
	CF set
Return: CF clear if successful
	    CX:DX -> previous INT 2F handler
	CF set on error
Notes:	This function is known to be supported by Paragon Technology Systems
	  PTS-DOS 6.51 and S/DOS 1.0, and probably was also supported
	  in earlier releases.  It is handled from within the kernel's
	  INT 2Fh dispatcher.
	Resident system extensions should call INT 2Fh/12F0h to intercept
	  the INT 2Fh chain. The main idea of this call is to allow them to
	  be moved to the HMA and intercept INT 2Fh without querying A20 state
	  and without interception of the INT2Fh vector. This is why the
	  "Chain2F" variable must be instanced by placing it to the SDA.
	On chain entry of the far procedure, CF must be set.  All registers
	  except for AX, BP, and DS remain unchanged. If a handler cannot
	  process the call, it must set CF and do a far jump to the previous
	  one in the chain in order to indicate an error.  If CF is still set
	  on exit, the call will be ignored.