The reason for adding this was that while testing mysqlbinlog on
a replication event with 3G event output, Linux failed reading
the whole file in memory with one read (only got 2G on first read
even if file had just been written).

- Don't reset info->error on write error in IO_CACHE.
- In case of write_error in IO_CACHE , always return -1
- Fixed wrong result from my_read when using MY_FULL_IO. Also don't give
  an error in case of retry.