Improve disk fragmentation of block files

This is a port of https://github.com/chronokings/huntercoin/pull/66.  The patch changes adding to the block files (blk????.dat) to be done in chunks of 16 MiB, which decreases file system fragmentation and is (I have heard) a big improvement especially on NTFS systems.

It has been tested already a while in Huntercoin, and I'm running a sync from scratch with this patch applied right now.  It correctly allocates the file in chunks, and is already at block 20k.  So it seems to work - but it would be cool to get more testing results, especially on Windows with NTFS.