rfc: drop support for historic fs formats from xfsprogs

[Christoph Hellwig]

While doing a lot of xfs_repair work I noticed that there's a lot of
effort spent on handling historic filesystem features.  The two that
make my life in repair hard are mostly:

 - unaligned inodes
 - dirv1

none of which every made it to production on Linux.  I'd like to suggest
to drop support for old filesystem features before a certain cut off
date after we release the currently pending xfsprogs 3.1.8 release.