Filesystem journaling. Reorganize the journal scan for the mountpoint to

Filesystem journaling.  Reorganize the journal scan for the mountpoint to
allow portions of the journal transaction to be written prior to the VOP
operation being executed, and accomodate transactional aborts if the VOP
fails.   Previously the journal records had to be written only after the VOP
succeeded.

Implement UNDO records, including writing out file data that is about to
be overwritten and the mtime prior to a write, rename, or remove.  UNDO
records allow us to create a reversable journal, where we can wind a mirror
forwards or backwards simply by scanning the journal forwards or backwards.