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	SetIntVector -- set a new handler for a system interrupt vector

	oldInterrupt = SetIntVector(intNumber, interrupt)
	D0			    D0         A1

	struct Interrupt *SetIntVector(ULONG, struct Interrupt *);

	This function provides a mechanism for setting the system interrupt
	vectors.  These are non-sharable; setting a new interrupt handler
	disconnects the old one.  Installed handlers are responsible for
	processing, enabling and clearing the interrupt.  Note that interrupts
	may have been left in any state by the previous code.

	The IS_CODE and IS_DATA pointers of the Interrupt structure will
	be copied into a private place by Exec.  A pointer to the previously
	installed Interrupt structure is returned.

	When the system calls the specified interrupt code, the registers are
	setup as follows:

	    D0 - scratch
	    D1 - scratch (on entry: active
			  interrupts -> equals INTENA & INTREQ)

	    A0 - scratch (on entry: pointer to base of custom chips
			  for fast indexing)
	    A1 - scratch (on entry: Interrupt's IS_DATA pointer)

	    A5 - jump vector register (scratch on call)
	    A6 - Exec library base pointer (scratch on call)

	    all other registers must be preserved

	intNum - the Paula interrupt bit number (0..14).  Only non-chained
		 interrupts should be set.  Use AddIntServer() for server
	interrupt - a pointer to an Interrupt structure containing the
		 handler's entry point and data segment pointer.  A NULL
		 interrupt pointer will remove the current interrupt and
		 set illegal values for IS_CODE and IS_DATA.

		 By convention, the LN_NAME of the interrupt structure must
		 point a descriptive string so that other users may
		 identify who currently has control of the interrupt.

	A pointer to the prior interrupt structure which had control
	of this interrupt.


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