Replace use of sizeof() in file import/export code with explicit byte count

[imaami]

Currently the binary layout of the weights files (and possibly other files) is defined using sizeof(int) and sizeof(size_t) which aren't guaranteed to be specific numbers across platforms. While sizeof(int) is most likely 4 bytes on any machine that Darknet is built for today, sizeof(size_t) can be either 4 or 8 bytes simply depending on whether the compiler is invoked with -m32 or -m64.

A binary file format should definitely be predictable, but unfortunately anyone who downloads the pre-trained weights files and attempts to run Darknet on a machine with a different build config than the author's will likely experience unexplainable behavior.

[adroit91]

I guess a pull request is in order.

[imaami]

@adroit91 You're right. I apologize that I haven't written a patch yet. I'll try to do that soon (and I hope @pjreddie starts merging my pull requests, there are several waiting already...)

[zinkcious]

Did @pjreddie accept your pull requests? @imaami