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>.

1 files changed, 1 insertions, 0 deletions

diff --git a/libman.h b/libman.h
index d29cfa36..283dafc7 100644
--- a/libman.h
+++ b/libman.h
@@ -23,6 +23,7 @@ enum	man_next {
 
 struct	man {
 	struct mparse	*parse; /* parse pointer */
+	const char	*defos; /* default OS argument for .TH */
 	int		 quick; /* abort parse early */
 	int		 flags; /* parse flags */
 #define	MAN_ELINE	(1 << 1) /* Next-line element scope. */