man(1): Fix override dir handling

Previously, override dir was affecting only some cases of manpath
determination.  Apply it only when all paths have been gathered instead.

Also look for override dir when sorting candidates.

Fixes src/tests/man-override-dir failing when --with-override-dir=od is
passed to ./configure.

[cjwatson: Refactored candidate comparison to use a strcmp-style
function, and simplified the addition of override directories a little
more.]

* src/man.c (compare_override_dir): New function.
(compare_candidates): Compare override directory status between
comparing section extensions and comparing locale elements.
* src/manp.c (insert_override_dir): Remove.
(get_manpath_from_path, add_man_subdirs): Remove calls to
insert_override_dir.
(create_pathlist): Add OVERRIDE_DIR at the canonicalization stage
instead.
* NEWS.md: Document this.

Reported-by: Nikola Forró <nforro@redhat.com>
Tested-by: Nikola Forró <nforro@redhat.com>