Support spaces in dataset names

[wlhlm]

While not explicitly documented, it is possible (and supported by core ZFS and utilities) to create ZFS datasets with spaces in them (see openzfs/zfs#1027 for a small discussion). zfs-periodic fails to support such datasets due to use of simplistic shell variable splitting using the default IFS which splits on spaces. Example from the code:

datasets=$(zfs list -r -H -o name $pool)
# ...
for dataset in $datasets; do

I've noticed this only after 4 years of using zfs-periodic that one of my ZFS filesystems wasn't snapshotting properly. 😅

Please consider supporting such datasets.

[ross]

PRs welcome. I don't really have a great setup atm for developing/testing zfs-periodic so it'll be a while before I'd get a chance to look at it.