MFC r272274:
Change the /var dataset in the default ZFS layout to have the
ZFS property canmount=off, making /var/db/pkg part of the / dataset, so
installed package files are consistent with the package database when
using ZFS boot environments (beadm).

PR:		193971
Reviewed by:	Shawn Webb, bcr
Approved by:	re (gjb), jmg
Relnotes:	yes
Sponsored by:	ScaleEngine Inc.