Handling of SION_BUFFERSIZE
Interestingly enough, the POSIX open call does not show this behaviour. So I changed to the POSIX interface. To get the buffering, I also added the 'buffered' option when opening the file. Is that option mature enough for production? I just have one remark: I had to set the environment variable SION_BUFFERSIZE to actually get the buffering to work. I think that is because of line 60 in sion_buffer.c, which shouldn't be inside the if-construct, so the buffer size gets set to the block size by default.