do_cmd_setshortversion():  Implementation for new markup.

convert_args():  New helper function.

do_cmd_optional():  Modified to understand when it is called from within
                    a convert_args() invocation.

do_env_funcdesc(),
do_env_funcdescni(),
do_cmd_funcline(),
do_cmd_funclineni(),
handle_classlike_descriptor(),
do_env_methoddesc(),
do_end_methoddescni(),
do_cmd_methodline(),
do_cmd_methodlineni():  Modified to use helper convert_args().

The convert_args() helper and related changes fix the generation of HTML
for the \optional markup, so that bogus </var>...<var> is not generated.
This closes SourceForge bug #113800.