Fix: generation of man pages with multiple refnames

Fix: generation of man pages with multiple refnames

In this proposed patch [1], we're trying to make a man page with two
command names in the NAME section, like this:

  tracef, vtracef - LTTng-UST printf(3)-like interface

Doing so causes the intermediary XML (what asciidoc outputs) to contain
this:

    <refnamediv>
      <refname>tracef, vtracef</refname>
      <refpurpose>LTTng-UST printf(3)-like interface</refpurpose>
    </refnamediv>

The refname is used by the docbook XSL to determine the output
filename.  This therefore results in an output file named
`tracef,_vtracef.3`, which is not desirable.

The problem is in the asciidoc pass, more specifically the part of
asciidoc.conf removed by this patch.  Is is there to override the
`source`, `version` and `manual` fields, but it also inadvertently
affects the refname generation.

Instead of playing with the asciidoc output template, this patch sets
the same `source`, `version` and `manual` attributes on the asciidoc
command line.  With the present patch applied, the XML output contains:

    <refnamediv>
        <refname>tracef</refname>
        <refname>vtracef</refname>
        <refpurpose>LTTng-UST printf(3)-like interface</refpurpose>
    </refnamediv>

The xmlto pass (docbook XML -> man page) then generates two files,
`tracef.3` and `vtracef.3`, where `vtracef.3` simply sources `tracef.3`
(which is what we want).

[1] https://lists.lttng.org/pipermail/lttng-dev/2020-February/029524.html

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>