libupkg: Add explicit conversions to stdio wrappers.

Some C libraries provide a macro version of feof which does not convert
the provided pointer to FILE * before dereferencing it.  Ordinarily,
this conversion would occur automatically since feof's prototype
specifies FILE *, but this is suppressed in the macro case.

It is not clear to me whether this behaviour is consistent with the
requirements of the C standard.  However, after encountering at least
two implementations which do this, explicit conversions seem simple
enough.