Add checks for Remote Backup system availability

[finzic]

In order to create cron jobs with the Backup script, this should be able to detect

• if the remote Backup server is available (ping)
• If it is possible to connect to it
• if it has the backup disk mounted (e.g. the ${DST_POOL} is available)
• if no pool is mounted, the script could do something like
  zpool import and check if the desired pool is ready to be imported
  zpool import <pool_name> to import the pool and then if everything is OK then continue the backup.
  If any of these goes wrong, the script shall exit gracefully with a self-explanatory message in syslog.

Doing this, one could simply turn on the Backup Server and the Backup Disk if it is external, then go away: the system would set up by itself.