SSH2: Continue Subsystem Implementation.

Removed unused piece of code @2352.
Added $this->_get_interactive_channel() to reset(). Unless we don't want to close the subsystem channel?? Leaving that decision up to you.
Merge pull request #178 from phonzie/patch-5

SSH2: Continue Subsystem Implementation.
Merge remote-tracking branch 'phpseclib/master' into php5

* phpseclib/master:
  SSH2: Continue Subsystem Implementation.