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>.

7 files changed, 20 insertions, 5 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. */
diff --git a/libmandoc.h b/libmandoc.h
index e5b52efd..71b9bb3f 100644
--- a/libmandoc.h
+++ b/libmandoc.h
@@ -66,7 +66,8 @@ void		 mdoc_addspan(struct mdoc *, const struct tbl_span *);
 void		 mdoc_addeqn(struct mdoc *, const struct eqn *);
 
 void		 man_free(struct man *);
-struct	man	*man_alloc(struct roff *, struct mparse *, int);
+struct	man	*man_alloc(struct roff *, struct mparse *,
+			const char *, int);
 void		 man_reset(struct man *);
 int		 man_parseln(struct man *, int, char *, int);
 int		 man_endparse(struct man *);
diff --git a/man.1 b/man.1
index 88ac444e..29771ea1 100644
--- a/man.1
+++ b/man.1
@@ -106,6 +106,9 @@ Override the default operating system
 for the
 .Xr mdoc 7
 .Ic \&Os
+and for the
+.Xr man 7
+.Ic \&TH
 macro.
 .It Fl h
 Display only the SYNOPSIS lines of the requested manual pages.
diff --git a/man.c b/man.c
index bc634341..8e8cf725 100644
--- a/man.c
+++ b/man.c
@@ -92,7 +92,8 @@ man_free(struct man *man)
 }
 
 struct man *
-man_alloc(struct roff *roff, struct mparse *parse, int quick)
+man_alloc(struct roff *roff, struct mparse *parse,
+	const char *defos, int quick)
 {
 	struct man	*p;
 
@@ -100,6 +101,7 @@ man_alloc(struct roff *roff, struct mparse *parse, int quick)
 
 	man_hash_init();
 	p->parse = parse;
+	p->defos = defos;
 	p->quick = quick;
 	p->roff = roff;
 
diff --git a/man_validate.c b/man_validate.c
index 5688bb54..48f2ed5b 100644
--- a/man_validate.c
+++ b/man_validate.c
@@ -379,6 +379,8 @@ post_TH(CHKARGS)
 
 	if (n && (n = n->next))
 		man->meta.source = mandoc_strdup(n->string);
+	else if (man->defos != NULL)
+		man->meta.source = mandoc_strdup(man->defos);
 
 	/* TITLE MSEC DATE SOURCE ->VOL<- */
 	/* If missing, use the default VOL name for MSEC. */
diff --git a/mandoc.1 b/mandoc.1
index 5b55b728..65487dba 100644
--- a/mandoc.1
+++ b/mandoc.1
@@ -85,6 +85,9 @@ Override the default operating system
 for the
 .Xr mdoc 7
 .Sq \&Os
+and for the
+.Xr man 7
+.Sq \&TH
 macro.
 .It Fl h
 Display only the SYNOPSIS lines.
diff --git a/read.c b/read.c
index 9b266090..acf1c1e5 100644
--- a/read.c
+++ b/read.c
@@ -298,7 +298,8 @@ choose_parser(struct mparse *curp)
 	/* Fall back to man(7) as a last resort. */
 
 	if (NULL == curp->pman)
-		curp->pman = man_alloc(curp->roff, curp,
+		curp->pman = man_alloc(
+		    curp->roff, curp, curp->defos,
 		    MPARSE_QUICK & curp->options ? 1 : 0);
 	assert(curp->pman);
 	curp->man = curp->pman;
@@ -696,7 +697,8 @@ mparse_end(struct mparse *curp)
 			curp->mdoc = curp->pmdoc;
 		else {
 			if (curp->pman == NULL)
-				curp->pman = man_alloc(curp->roff, curp,
+				curp->pman = man_alloc(
+				    curp->roff, curp, curp->defos,
 				    curp->options & MPARSE_QUICK ? 1 : 0);
 			curp->man = curp->pman;
 		}
@@ -940,7 +942,8 @@ mparse_alloc(int options, enum mandoclevel wlevel, mandocmsg mmsg,
 		    curp->roff, curp, curp->defos,
 		    curp->options & MPARSE_QUICK ? 1 : 0);
 	if (curp->options & MPARSE_MAN)
-		curp->pman = man_alloc(curp->roff, curp,
+		curp->pman = man_alloc(
+		    curp->roff, curp, curp->defos,
 		    curp->options & MPARSE_QUICK ? 1 : 0);
 
 	return(curp);