path-util: also support ".old" and ".new" suffixes and recommend them

path-util: also support ".old" and ".new" suffixes and recommend them

~ suffix works fine, but looks to much like it the file is supposed to be
automatically cleaned up. For new versions of configuration files installers
might want to using something that looks more permanent like foobar.new.
So let's add treat ".old" and ".new" as special.

Update test to match.