Resolve return value and timeout for multi-command and continuation prompts in replwrap

* for multiple commands, such as in
  ``'\n'.join((cmd, cmd2))``, cmd2 and beyond
  previously used a ``timeout`` of 1, instead of
  the specified or default ``timeout``.
* furthermore, the output of multi-command output
  was discarded, only the last-most command output
  was received.  Resolved by joining the result of
  ``self.child.before``.
Yes, u''.join is more efficient than += for many (especially older) versions of python, we can change that easily.