- exit right below 6502 vectors.
- keep exit addr stable as it may be called from asm.

Although the primary target OS for the Apple II for sure isn't DOS 3.3 but ProDOS 8 the Apple II binary files contained a DOS 3.3 4-byte header. Recently I was made aware of the AppleSingle file format. That format is a much better way to transport Apple II meta data from the cc65 toolchain to the ProDOS 8 file system. Therefore I asked AppleCommander to support the AppleSingle file format. Now that there's an AppleCommander BETA with AppleSingle support it's the right time for this change.

I bumped version to 2.17 because of this from the perspective of Apple II users of course incompatible change.