Add a pre-world mode of updating similar to the -p option that can be
passed to mergemaster.  In this mode, only changes to /etc/master.passwd
and /etc/group are merged to /etc.  In addition, it uses a temporary
tree to stage these changes rather than overwriting the existing
'current' and 'previous' trees so that a full update can be run after
a normal installworld has completed.

MFC after:	2 weeks
Add VM_LAST, a special last element in enum VM_GUEST and use it in CTASSERT
to ensure that vm_guest range is covered by vm_guest_sysctl_names.

Suggested by:	mjg