bug#36927: 26.2.90; include stdarg.h is without hyperlinking on vfprintf's manpage

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 05 Aug 2019 19:10:28 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 36927@debbugs.gnu.org
> 
> > From: VanL <van@scratch.space>
> > Date: Mon, 5 Aug 2019 11:51:56 +1000
> > 
> > See manpage for vfprintf().
> > 
> > Unlike the first instance of the #include preprocessing directive
> > 
> >   #include <stdio.h>
> > 
> > at approximately line 12 of printf(3), which is underlined and
> > hyperlinked, the second instance at approximately line 32, is not
> > underlined and not hyperlinked.
> > 
> > 1. M-x man RET vfprintf
> > 2. on line 32 see #include <stdarg.h> which is without hyperlinking
> 
> For some reason, we don't put a button on the second #include.

The logic in Man-highlight-references0 was flawed: it assumed that a
section ("SYNOPSIS", "FILES", "SEE ALSO", etc.) is always received
entirely in a single chunk of text that is read from the subprocess,
which is, of course, not guaranteed, and depends on factors such as
the size of each section, buffering, how busy is the system, etc.
(Those reasons also made this a bitch to debug, because a bug comes
and goes unpredictably, and never shows when one steps with Edebug.)

I think I made it much better on master, although I still see a rare
case where one or the other "include" is not buttonized.  But I've run
out of time, so someone else will have to find and fix what I missed.

I'm leaving this bug open because I don't think it's 100% fixed.