Reset process suggestion from core repo

[rdpeake]

Tranferring an issue from the core branch for handling reset from within freezer menu:
MEGA65/mega65-core#72

[gardners]

Both bug and enhancement because it combines improvements, as well as correcting behaviour to match user expectations, in particular, making sure that there is a valid ROM that can be used.

1. Check for cartridge signature at $8000 and prompt for removal before reset
2. Check for ROM version signature, and prompt to load a ROM first if no (longer) present.
3. Re-enable ROM write-protect before completing reset

[REDACTED] is a convenient test as to whether the reset process works or not, as it currently crashes.

[gardners]

Should the process simply force loading of MEGA65.ROM? Quite possibly.