MFC r287964:
Kernel part of reroot support - a way to change rootfs without reboot.
Note that the mountlist manipulations are somewhat fragile, and not very
pretty. The reason for this is to avoid changing vfs_mountroot(), which
is (obviously) rather mission-critical, but not very well documented,
and thus hard to test properly. It might be possible to rework it to use
its own simple root mount mechanism instead of vfs_mountroot().
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D2698
git-svn-id: svn://svn.freebsd.org/base/stable/10@293743
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