This changes the signatures for FILE::seek and FILE::tell, to use
`off_t` as they use lseek internally. `fpos_t` is also redefined to use
`off_t`.
Dr. POSIX says that fpos_t is:
> A non-array type containing all information needed to specify uniquely
> every position within a file.
In practice, most *NIX typedef it to `off_t`, or a struct containing an
`off_t` and some internal state.
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