Handle warnings and parallell backups.

Better handling of warnings.

* backup-all: New option: --retry.  Only run df on the specified
  partitions.  Added handling of a "hold" file.

* backup-one-task: New option: --retry.  Added -warn, -nowarn and
  -neverwarnfree status files.

* lysrdiff-status: Print number of warnings per partition, and list
  all warnings.

Support starting two "backup-all" at the same time.

* distribute-tasks
  (write_task_lists): New argument: partitions.  Only output data for
    the specified partitions.  Create a lock directory to ensure that
    two distribute-tasks processes cannot clobber the same file.
  (main): Expect partition number on the command line.

Minor.

* Makefile (install-one-task): New target, extracted from install.