fsync - Add sysctl to relax fsync requirements.

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).