Progress indication, stop and lock files, and more.

Progress indication.  Lock files.  More output.  Run ssh in batch
mode.  Support --failed.  Support a stop file.

* backup-one-task: New argument: msg.  New optional argument --failed.
  Default remoterdiff to /opt/LYSrdiff/bin/rdiff-backup, and add
  special cases for sedesopim, home and uhumhummy.  Remove special
  case for stalingrad.  Handle exclusion files.  Create
  rdiff-backup.out and backup-summary.txt.  Lock the result directory.
  Unset SSH_ASKPASS and SSH_AUTH_SOCK.  Run ssh in batch mode, to
  avoid password prompts.  Print status message including ETA while a
  backup runs.  Use the --force argument to rdiff-backup to make it
  easier to recover from an initial failure.

* backup-all: New optional argument --failed.  Count the number of
  jobs to run, and pass a running total as the msg argument of
  backup-one-task, to get a nice progress indication.  Stop cleanly if
  the file /nobackup/backup.lysator/etc/stop is found.