Fix a bug in the move-machine script
The generation of the command to unexport NFS filesystems could generate an invalid command. Leading spaces were not being stripped, and in cases where there is more than one backup target for a machine we need to unexport every target. Because we also had 'set -e' in operation at this point, the script would fail there and never clean up the moved ZFS. I don't mind if we fail to unexport; if that's subsequently a problem for removing the ZFS then the script will fail at that point. This change makes the script generate better exportfs -u commands and not exit if they fail.