rework getFileSize

I have been rewriting getFileSize with the purpose of avoiding opening the encrypted file as an encrypted stream. I will need this to avoid recursion when I rework #8390 (to make the encrypted stream seekable the unencrypted filesize is required upon opening the stream).
I used the opportunity to make the procedure more robust for a number of exotic situation, such as local streams that are non-seekable or streams that return less than 8192 bytes upon fread. That should all be covered now.
I didn't do much testing yet. It seems to work on my installation. Any feedback appreciated.
I would favour a separate PR for that, provided that the 6126 will be featured also in other places that should be updated accordingly.