use statfs() to detect file alignment when --enable-stat-at-open is not set
Right now we set alignment to -1 if we aren't able to stat the file at open time. As an alternative, we can call statfs() on all mounted file systems to get the default block size. This can be done on rank 0 at startup and broadcast to all processes.
We also need to add an exception for Lustre, because it appears to always set the block size in statfs() to 4K.