Add list-old-{dirs,files,libs} targets.

These targets generate a raw list of the candidate old files roughly
corresponding to the values of OLD_DIRS, OLD_FILES, and OLD_LIBS.
Currently list-old-files also includes uncompressed manpages in
addition to compressed manpages.

Use these targets in the implementation of check-old-* and
delete-old-* to replace duplicated logic.

Reviewed by: imp, emaste
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33327