Revert "openbsd-man: removed; superseded by mdocml-1.13.2."

This reverts commit cb9eef68c7bce40a9424fc35ed47f2fe3856cd4c.

Re-add this again, looks like it needs to be there for a while.
mdocml: do not use supplied man(1).

The mdocml man(1) implementation needs the sqlite db to search for manual pages,
and if it's outdated it won't find the matching manpage.

Disable man(1) and related components and switch back to openbsd-man again.
openbsd-man: require mdocml>=1.13.2_5.