Add support for up to 9 backup slurmctld

Specify "BackupController#=<hostname> and "BackupAddr#=<address>" to
  identify up to 9 backup servers in slurm.conf.
Output format of "scontrol ping" and the daemon status at the end of
  "scontrol status" is modified to report up status of the primary and all
  backup servers.
"scontrol takeover [#]" command can now identify the BackupController
  index number. Default value is "1" (the configured "BackupController" or
  "BackupController1" node).