freebsd-update: improve BE creation feature

This addresses one nit and one bug in the BE creation feature of
freebsd-update:

The nit addressed is that it currently only names the BEs after the
userland version, but the kernel version may be higher. After this
change, we request both and pass them through sort(1) to choose the
highest. This is especially helpful if a freebsd-update patch touched
one but not the other.

The bug fixed is that roots updated that are not located at '/', e.g.,
by using -b or -j, will no longer create boot environments
automatically. There's a very low chance these will actually change the
BE in any meaningful way, anyways. It could make sense in the future
to allow an argument-override to create the BE anyways if someone comes
up with a non-standard setup, e.g., where a jail is an important part of
their boot environment on an appliance or some such setup.

Half of this patch is submitted by delphij@, the other half kevans@.

PR: 261446
MFC after: 3 days
Reviewed by: delphij, emaste, Dave Fullard <dave_fullard.ca>
Differential Revision: https://reviews.freebsd.org/D34257