[$] Support for atomic block writes in 6.13

Atomic block writes, which have been discussed here afew times in the past, are block operations that either complete fully ordo not occur at all, ensuring data consistency and preventing partial (or"torn") writes. This means the disk will, at all times, contain either thecomplete new data from the atomic write operation or the complete old datafrom a previous write. It will never have a mix of both the old and the newdata, even if a power failure occurs during an ongoing atomic writeoperation. Atomic writes have been of interest to many Linux users,particularly database developers, as this feature can provide significantperformance improvements.