Fix not being able to run multiple intances of the same jail, using different porttree/sets

This is fixed by including the MASTNAME into the ZFS target dataset. The mount
dir already had the porttree/setname as part of it, but the ZFS dataset
was always just doing JAILNAME/ref instead of JAILNAME-ptname-setname/ref, ie
MASTERNAME/ref.

To avoid needing a useless base dir of MASTERNAME/ref, just use MASTERNAME-ref