Valgrind reports uninitialised values sent to system call in legacystore

Running the legacystore tests under valgrind highlight that it doesn't initialise all the values it sends to disk.

Even if these bytes are actually never used when the store is read, having this behaviour makes it hard to use valgrind to check that the store doesn't have other problems.

There is a compile time option to enable initialising of the full written buffer, I think it is wise to enable this option always, so that testing using valgrind is productive.