Fix doublequotes in NETFS

Jeremy Laidmann reported a bug in /usr/share/rear/restore/NETFS/default/40_restore_backup.sh

>>The error in line 42 of 40_restore_backup.sh appears to be expecting $BACKUP_TYPE to be non-empty.  I 
>>think the default is "full" and so I suspect this can be fixed by enclosing "$BACKUP_TYPE" in double quotes.  
>>If I set BACKUP_TYPE to "full" in my site.conf then this message doesn't show up.  I can't tell if this is 
>>causing other problems like...