Code cleanups in libwebhdfs.
- don't duplicate exception.c, exception.h, expect.h, jni_helper.c. We have one copy of these files; we don't need 2.
- remember to set errno in all public library functions (this is part of the API)
- fix undefined symbols (if a function is not implemented, it should return ENOTSUP, but still exist)
- don't expose private data structures in the (end-user visible) public headers
- can't re-use hdfsBuilder as hdfsFS, because the strings in hdfsBuilder are not dynamically allocated.