Improvement to paging logic

[haerfest]

In a thread on the Stardot forum it was revealed that Electron OS 1.00 expects SHEILA &FE05, used to both clear interrupts and page in sideways RAM/ROM, to only page when all interrupt-clearing bits are zero, i.e. &0x. Most expansions that support sideways RAM/ROM, such as the Plus 1, implement logic to that effect.

Elkulator currently honours every paging request, which in specific circumstances could lead to the OS inadvertently paging in bank #0 when servicing interrupts.

Probably placing the code in ula.c lines 249-255 inside a guard is sufficient:

if ((val & 0xF0) == 0x00)
{
  // existing paging code
}

[uiopaubo]

I've extended this logic in the pull request for this topic, hopefully capturing the proper hardware behaviour.