lowlevel-z80: various (important) fixes

lowlevel-z80: various (important) fixes

1.	Exit with reti, most platforms don't care but a few do so avoid
	digging holes

2.	MSX2 is the first platform we handle where not servicing an IRQ
	jams you in a loop. Provide a hook to always service some interrupts
	even if we are on an IRQ stack or otherwise detained from calling
	platform_interrupt

3.	The di() call saved the flags but forgot to DI, this fixes several
	rather odd bugs I'm sure

4.	The signal processing on irq return is suspect, note it for future
	sorting out.

With this and the MSX2 fixes we get closer to a basic MSX2 boot