Merge changes fom master and add workaround for DASH shell problems

[stuehmer]

No description provided.

[stuehmer]

I admit the /bin/bash change is a portabilty problem for the leecallan branch. However, before this workaround, the branch was broken using dash shell (e.g. on debian). I suggest while the script uses bash features we restrict the script to bash shell.

[rlaager]

@highvoltage What is this "leecallen" branch?

[stuehmer]

It is a branch where the snapshot names (e.g. daily, weekly, ...) are NOT part of the snapshot file name together with the date but rather in a ZFS property. This way all snapshots can be managed more homogeneously purely by their date. This is necessary e.g. when exposing them to Samba as Shadow Copies.
(Nowadays Samba has provided some workarounds to ignore variable parts of the snapshot name but this is only available in newer Samba versions and can be fixed here just as well.)