Make the Processor Status register match a real 6502 at power-on.

When describing the CPU's reset pin, the W65C02S data sheet says:

> All Registers are initialized by software except the Decimal and Interrupt
> disable mode select bits of the Processor Status Register (P) are initialized
> by hardware.

It then has a diagram of the power-on state of the processor status register:

>     7 6 5 4 3 2 1 0
>     * * 1 1 0 1 * *
>     N V - B D I Z C
>
> * = software initialized

Confusingly the text indicates that only the D and I flags are initialised by
hardware, while the diagram indicates that the B flag is initialised too.

Meanwhile, https://www.nesdev.org/wiki/CPU_power_up_state says that
the power-on state of the NES CPU is $34 (exactly matching the diagram above)
but https://www.nesdev.org/wiki/Status_flags#The_B_flag says that the B flag
does not physically exist within P register, it's only relevant in the copy
of P that gets pushed to the stack by BRK (set), PHP (set), or an interrupt
signal (cleared).

As a result, the most sensible power-on state for the processor status register
is with the "interrupt disable" flag set and everything else cleared.

src/main/java/com/loomcom/symon/Cpu.java

@@ -126,7 +126,7 @@ public class Cpu implements InstructionTable {
         // Clear status register bits.
         state.carryFlag = false;
         state.zeroFlag = false;
-        state.irqDisableFlag = false;
+        state.irqDisableFlag = true;
         state.decimalModeFlag = false;
         state.breakFlag = false;
         state.overflowFlag = false;