".TS H" macro

[Kristaps Dzonsons <kristaps@bsd.lv>]

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Hi!

A recent issue for lowdown[1] noted the existence of ".TS H", which 
allows for multi-page PDF TS/TE tables.  These don't affect mandoc, 
where the PDF tables aren't great anyway (that's my fault from way long 
ago), so using ".TS H" instead of ".TS" is ignored and fine.

The problem is that this introduces a new macro to signal the end of the 
table header, which groff will re-issue at the start of a new page.  And 
of course, the macro is ".TH".

.TH "Untitled article" "7" ""
.TS H
tab(|) expand allbox;
lb lb
l l.
T{
app
T}|T{
quality
T}
.TH
T{
foo
T}|T{
bar
T}
.TE

In mandoc, the subsequent ".TH" overrides the main document's ".TH", so 
the title and stuff shows up as empty.

I've included a patch that has the TH parser bail if it already 
encounters a title.  This passes "make regress", but it may be more 
appropriate to check if it's in a TS/TE context.

Best,

Kristaps

[1]
https://github.com/kristapsdz/lowdown/issues/65

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Index: man_validate.c
===================================================================
RCS file: /home/cvs/mdocml/mandoc/man_validate.c,v
retrieving revision 1.155
diff -u -p -u -r1.155 man_validate.c
--- man_validate.c	30 Oct 2020 13:24:33 -0000	1.155
+++ man_validate.c	28 Mar 2021 11:21:33 -0000
@@ -475,6 +475,9 @@ post_TH(CHKARGS)
 	struct roff_node *nb;
 	const char	*p;
 
+	if (man->meta.title != NULL)
+		return;
+
 	free(man->meta.title);
 	free(man->meta.vol);
 	free(man->meta.os);