Documentation: option to render literal text as bold for manpages

Documentation: option to render literal text as bold for manpages

This allows manpages viewed on a tty to render inline literal
text in a manner that is distinct from the surrounding text.

The initial implementation (pre-mailing-list) of this patch
included a conditional variant of the XSLT code in
manpage-base.xsl and use xmlto's --stringparam option to
optionally enable the functionality. It turns out that
--stringparam is broken in all versions of xmlto except for the
pre-release, SVN version. Since xmlto is a shell script the patch
to fix it is simple enough, but I instead opted to use xmlto's
"module" functionality.

Testing done with asciidoc 8.3.1 and docbook-xsl 1.74.0.

Signed-off-by: Chris Johnsen <chris_johnsen@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>