Fix lldb -P on Linux
- now prints the correct PYTHONPATH
- update dotest.py to use lldb -P result correctly
- resolves TestPublicAPIHeaders test failure (on Linux)



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@171558 91177308-0d34-0410-b5e6-96231b3b80d8