rc.d/jail: add legacy compatibility for zfs.dataset

Evaluate the jail_${jailname}_zfs_dataset variable for legacy
jail managers.

This variable can take a space separated list of datasets.
The singular was used specially to allow unmaintained jail
managers like ezjail to use this (simply rename
jail_${jailname}_zfs_datasets in the ezjail config to
jail_${jailname}_zfs_dataset).