[unsafe] Replace fs::File with libc::FILE

[lucic71]

Currently we translate FILE* as *mut std::fs:FILE. This makes it difficult to translate functions such as feof and ferrror.

feof and ferror both peek the internal state of the FILE object. std::fs::File doet not keep an internal state, it propagates the error right at the point of the usage, for example when performing a read or a write. On the other side, *mut libc::FILE, which is the direct FFI equivalent of FILE*, keeps the internal state, making the translation of feof and ferror easy.

Another advantage of switching to *mut libc::FILE is that it simplifies all stdio.h rules. For example fwrites becomes a simple call to libc::fwrite, instead of the hand-rolled implementation that we have now for writing a buffer into std::fs::File.