[BUG] Altfan uses unsupported timer mode

[vintagepc]

Describe the bug
Altfan uses mode 9 on timer 4. This is unsupported by SimAVR.

To Reproduce
Steps to reproduce the behavior:
Run altfan firmware with -vvv
Observe: TIMER: avr_timer_write_ocr-4 mode 0 UNSUPPORTED

Expected behavior
A functional mode 9

Additional context
Maybe roll this in with a fix for the missing PC PWM mode...?? (#75 )

[vintagepc]

Casually poking at this, it looks like the implementation should be fairly straightforward with existing timer code as reference.
The only thing I'm struggling to understand is how the specific updating of TOV or OCRs is controlled. It's a little clearer in the external clocking function but I can't make heads nor tails of the internal clock configuration for a timer.