PKG_INTR, Register interrupt handler

rst $08, defw $180f


IN:	A=reason code;
		int_prc, register single process interrupt routine
		int_pkg, register global package interrupt call
	C=type of interrupts to accept (set bits required);
		0: TICK (1/100s)
		1: SEC (1s)
		2: MIN (1m)
		3: UART
	B=# of acceptable interrupts required before running handler
	(eg set C=1, B=5 to run every 5 TICKs)
	HL=routine address (A=int_prc) or package call ID (A=int_pkg)
OUT(success):
	Fc=0
OUT(failure):
	Fc=1
	A=rc_room, no room to install handler
	A=rc_unk, unknown reason code

Registers changed after return:
  ......../IXIY same
  AFBCDEHL/.... different

Notes:
	If a handler for the same process or package is already installed,
	it will be replaced by the new handler specified.
	Single process interrupts are only serviced when the process is
	active *and* when no OZ or package call is in progress.
	UART interrupts are included for completeness only; no information
	is available on how to use them.
