run-parts: support uapi configuration_files_specification
https://uapi-group.org/specifications/specs/configuration_files_specification/
This allows users of run-parts to implement the scheme where packages install files into /usr and the admin can override these files by placing a file of the same name in the appropriate directory in /etc.
This extends the run-parts interface, allowing more than one directory to be passed on the command line. Content in earlier directories override content in later directories. That is, you might want to call run-parts like this:
run-parts --report --exit-on-error /etc/foo.d /usr/lib/foo.dCloses: #1084170