fsync - Add sysctl to relax fsync requirements.
* sysctl vfs.hammer.fsync_mode (defaults to 0 == full fsync semantics).
0 - full fsync semantics
1 - asynchronous
2 - synchronous fsync on close if fsync called prior to close
3 - asynchronous fsync on close if fsync called prior to close
4 - ignore fsync (30-second system sync takes care of it)
* This is likely a temporary measure until HAMMER gets a REDO log.
It is mainly to facilitate testing and to reduce the pounding
disks take from pkgsrc bulk builds (pkg_add seems to insist on
calling fsync() a lot for no reason).