added the configuration item BCONFIG_CAN_OVERWRITE, which specifies whether the

added the configuration item BCONFIG_CAN_OVERWRITE, which specifies whether the
caller is allowed to perform the following 0-copy write operation:

1. call bdirect_read_prepare()
2. modify the data directly in the buffer returned (e.g. put \0's there)
3. call bflush() before any seek operation to let the fastbuf know