Make it possible to reroot into NFS. This means one can have
eg an NFSv4 root over WiFi: boot from md_root (small rootfs image
preloaded by loader(8)), setup WiFi, and then reroot into the actual
root, over NFS.

Note that it's currently limited to NFSv4, and due to problems with
nfsuserd(8) it requres a workaround on the server side: one needs
to set the vfs.nfsd.enable_stringtouid=1 sysctl and not run nfsuserd(8)
on either the server or the client side.

Reviewed by: rmacklem@
MFC after: 1 month
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6347