@justinmk `os_get_file_size()` wants a path to a file instead of a file descriptor.
`os_file_get_info_fd()` could be used instead, but then this would take 3 LOC.
Also I'm not sure if seek and stat on a file descriptor are consistent with each other.
So I decided to leave this `lseek()`-version because this should be simple and clean enough.