This patch produces the same usage as before, but I have rewritten _format_actions_usage() for both HelpFormatter and MultiGroupFormater.

The original HelpFormatter._format_actions_usage() formats the actions, splices in group markings, cleans up the text, if needed, tries to break it down into parts.  But this is fragile, as shown here and in issues 11874, 18349).

Now _format_group_usage() and _format_just_actions_usage() format groups and actions directly, without the splice and divide steps.  _format_actions_usage() for both classes call these to build a list of usage parts.

This change also solves http://bugs.python.org/issue11874 and http://bugs.python.org/issue18349, since it does not have to break up a formatted text line (and in the process get confused by [] and ()).