Handle MOVED_LIBS in list-old-files

MOVED_LIBS is used when a library moves from one directory to another,
e.g. /usr/lib/libc++.so.1 to /lib/libc++.so.1.

delete-old-files and delete-old-libs are two separate steps, so that
old libraries can be retained until third party software packages are
rebuilt or reinstalled.

Having two copies of the same shared library with the same so version
(as can happen when delete-old-libs hasn't been run) causes trouble.
The PR below gives one example.

Libraries listed in MOVED_LIBS are logically equivalent to updating a
library without changing the so version, and should be removed as soon
as possible. Handle them in list-old-files and thus delete-old-files.

Leave them also in *-old-libs for now, in case the user updates their
tree between running delete-old-files and delete-old-libs.

PR: 272642
Reviewed by: dim
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Co-authored-by: Dimitry Andric <dim@FreeBSD.org>
Differential Revision: https://reviews.freebsd.org/D42197