Implement `atari` `fgetpos` and `fsetpos` using `NOTE`/`POINT`

[mysterymath]

The Atari DOS NOTE/POINT functionality isn't expressive enough to implement fseek, since the former use sector/offset, while the latter uses byte offsets from start/end of file. It is expressive enough to implement fgetpos and fsetpos, since these operate on an opaque fpos_t value, which could be interpreted as a sector/offset pair.