INT 21 - LANtastic - SET MESSAGE SERVICE VECTOR                                 
	AX = 5FE3h
	ES:BX -> FAR routine for processing network messages
Return: CF clear if successful
	CF set on error
	    AX = error code
Notes:	handler must chain to previous handler as its first action
	on invocation, ES:BX -> just-received message
SeeAlso: AX=5FE2h