INT 21 - DOS 1+ - SET INTERRUPT VECTOR

	AH = 25h
	AL = interrupt number
	DS:DX -> new interrupt handler
Notes:	this function is preferred over direct modification of the interrupt
	  vector table
	some DOS extenders place an API on this function, as it is not
	  directly meaningful in protected mode
	under DR DOS 5.0-6.0, this function does not use any of the
	  DOS-internal stacks and may thus be called at any time; however,
	  under Novell DOS 7.0 - DR-DOS 7.02, this function was not reentrant.
	  Since 1998/05/29, DR-DOS 7.03 no longer uses any internal stacks and
	  tests for this function much earlier, to allow a minimal stack usage
	  of just two words in addition to the IRET frame, allowing it to be
	  called from INT 21h functions, specificially device drivers.  This
	  fixes the MCS SMB client
	Novell NetWare (except the new DOS Requester) monitors the offset of
	  any INT 24 set, and if equal to the value at startup, substitutes
	  its own handler to allow handling of network errors; this introduces
	  the potential bug that any program whose INT 24 handler offset
	  happens to be the same as COMMAND.COM's will not have its INT 24
	  handler installed
SeeAlso: AX=2501h,AH=35h