Change the /var dataset in the default ZFS layout to have the
ZFS property canmount=off so that /var/db/pkg and other such directories
are part of the / dataset, and only /var/mail, /var/log, and /var/crash
are excluded from the ZFS boot environment (beadm).

PR:		193971
Approved by:	jmg
MFC after:	ASAP
Relnotes:	yes
Sponsored by:	ScaleEngine Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272274 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f