Fixed b18655: cleaned up script interpreter file reference handling.

This change addresses shutdown crashes in the python lldb module when
the script interpreter was hanging on to saved file references after
leaving a session.  It also gets rid of extra references to the
stdin/stdout/stderr python file objects that are created when entering
the session.

This change also moves the bundled pyexpect 2.4 library to the front
of the python library path so that a python distribution default
pyexpect (2.3 in Ubuntu 12.04) is not picked up first.