Amiga: Kill the legacy CIA emulation and fix Agnus ID for A2000 and
A500.

CIA changes:
- Add missing virtual keyword to actually call the right TOD clock
- Outputs depend on the input state (fixes A1000 overlay switch)
- Switching to serial output mode with one-shot activated causes a pulse
on SP (correct? needed for keyboard handshake)
- Serial input is clocked by the CNT input, not the default clock
- Mask offsets to 4 bits
- Writing the Timer High register with one-shot activated immediately
causes the timer to be latched and the timer started, regardless of
timer start value

Curt: Please review those changes.