mdoc(7) already uses the mandoc(1) -Ios argument in the footer line

mdoc(7) already uses the mandoc(1) -Ios argument in the footer line
when .Os has no argument, so do the same for man(7) when .TH has less
than four arguments; there is no reason to treat both differently.
Issue found following a question from Thomas Klausner <wiz at NetBSD>.