Port atari Sparta DOS X `lseek` from cc65

[mysterymath]

The cc65 lseek (and thus, fseek) for atari only works on Sparta DOS X, since the NOTE/POINT system available on other DOS'es are insufficiently expressive to implement it (since they speak Sectors and Byte offsets, rather than offsets from file begin/end). This limits its usefulness compared to the rest of the atari POSIX file compat library.