Move zfsd's case files to /var/db/zfsd/cases

It was never really appropriate to store these thing in /etc

cddl/usr.sbin/zfsd/case_file.cc
Change the case file location

etc/mtree/BSD.root.dist
Don't create /etc/zfs/cases

etc/mtree/BSD.var.dist
Do create /var/db/zfsd/cases

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