Fix zfs-mount-generator for datasets with spaces

Fix zfs-mount-generator for datasets with spaces

Alternative implementation of @rlaager's original modification
of zfs-mount-generator fix, with @chrisrd's comments. Set
IFS to be only the tab character, matching our `-H` call in
`zfs list`, allowing spaces to appear in dataset names (and
mountpoints).

Also adds comments explaining our rationale.

Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #8708
Closes #8718