- steal from the reserved bits instead of the bitmap bits, another

- steal from the reserved bits instead of the bitmap bits, another
  piece of sage Andreas advice
- add an llh_flags field to the header
- rename confusing lgh_index to lgh_last_idx
- if the log is marked ZAP_WHEN_EMPTY, the log is full, and all of the
  records are cancelled, call llog_destroy from llog_cancel_rec
- extend llog_open to also support reopen-by-name
- add llog_vfs_read_blob and llog_vfs_read_header
- removed a bunch of unnecessary EXPORT_SYMBOLs
- add llog_lvfs_open()