Extend LBR so that it includes the control-flow on a taken interrupt
Add a new command 'brkint <0|1>' to support Break on Interrupt
Internal: in core emulation loop, moved IRQ/NMI check to start of loop so that just the "interrupt vectoring" case can be single-stepped (instead of previously opcode + interrupt vector).
Debugger help chm: update Breakpoints section to include BRK, BRKOP and BRKINT