cp: fix some cases with infinite recursion

As noted in the PR, cp -R has some surprising behavior. Typically, when
you cp -R foo bar where both foo and bar exist, foo is cleanly copied
to foo/bar. When you cp -R foo foo (where foo clearly exists), cp(1)
goes a little off the rails as it creates foo/foo, then discovers that
and creates foo/foo/foo, so on and so forth, until it eventually fails.

POSIX doesn't seem to disallow this behavior, but it isn't very useful.
GNU cp(1) will detect the recursion and squash it, but emit a message in
the process that it has done so.

This change seemingly follows the GNU behavior, but it currently doesn't
warn about the situation -- the author feels that the final product is
about what one might expect from doing this and thus, doesn't need a
warning. The author doesn't feel strongly about this.

PR: 235438
Reviewed by: bapt
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D33944