* Backup srcdir with rolling .orig extension
* Backup srcdir with rolling .orig extension

  If the source directory needs to be backed up, create it in the tmpdir
  with a .orig extension.  If that already exists, progressively add
  integer extensions (.1, .2, etc.) until a free path is found.

  New function:
    backup_filename: Construct the next path name to put the backup.

csdms/bocca-tools