In order to achieve this, root filesystem mount is moved from
SI_ORDER_FIRST to SI_ORDER_SECOND in the SI_SUB_MOUNT_ROOT sysinit
group. Now, modules which wish to usurp the default root mount
can use SI_ORDER_FIRST.
A compiled-in or preloaded MFS filesystem will become the root
filesystem unless the vfs.root.mountfrom environment variable refers
to a valid bootable device. This will normally only be the case when
the kernel and MFS image have been loaded from a disk which has a
valid /etc/fstab file. In this case, the variable should be manually
overridden in the loader, or the kernel booted with -a. In either
case "mfs:" should be supplied as the new value.
Also fix a typo in one DFLTROOT case that would not have compiled.
88d4183b84