Since gzip uses 'printf-posix' can't offsets be printed using that
instead of fprint_off? Patch attached + 1 more removing #ifndef CHAR_BIT
cruft.

Also, now that POSIX-1.2024 requires strcpy to be async-signal-safe can
'volatile_strcpy' be removed? Or is there something more to
that. Signals hurt my brain a bit, sorry...

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