[backup/restore] unescaped environment variables

[shutterfreak]

The backup script doesn't escape some environment variables when invoked without arguments. See https://community.openhab.org/t/backup-restore-scripts-for-linux-and-macos/34491/44?u=shutterfreak

Offending code:

    echo "Set $OPENHAB_BACKUPS to change the default backup directory."
    echo "Set $OPENHAB_BACKUPS_TEMP to change the default backup temporary directory."

Suggested fix:

    echo "Default backup directory: '$OPENHAB_BACKUPS'"
    echo "Set \$OPENHAB_BACKUPS to change the default backup directory."
    echo "Default backup temporary directory: '$OPENHAB_BACKUPS_TEMP'"
    echo "Set \$OPENHAB_BACKUPS_TEMP to change the default backup temporary directory."

I'd also add that the backup script must be run as root (can be checked with $UID (0 for root) when the script is run without arguments.

[shutterfreak]

Can't get to comply with DCO checks in my PR though.