This CR is created to track the addition of -z option to umountall(1M).
This change was originally part of 6502014, but it is more logical to
create a separate CR just for the -z option.
-z will be used to fix stop methods of various smf services:
$ grep umountall /lib/svc/method/*
/lib/svc/method/nfs-client: /sbin/umountall -F cachefs
/lib/svc/method/nfs-client: /sbin/umountall -F nfs
/lib/svc/method/smb-client: /sbin/umountall -F smbfs
/lib/svc/method/svc-autofs: /sbin/umountall -F autofs
they should use the -z flag. e.g. /sbin/umountall -F nfs -z