Move pbc.* to pbcutil/

- Remove unnecessary includes of pbc.h.
- Reorder some of the includes in C++ code manually; elsewhere, just a
  manual replacement of the include paths.
- Some minor cleanup.

This introduces a cyclic fileio<->pbcutil dependency, but that needs to be solved separately.

Change-Id: I13becc82aaa217288ea45df72bb6198377de9d0f