Bugfix for mismatched id sizes when reading xdr

This branch is originally from Nov 2013!  I didn't want it to get lost forever, so I rebased it and made a PR.  @roystgnr, I know buildbot is giving you headaches right now, so can you let us know the configure options you use in your "BigIDs" tests?

This fixes a regression in our BuildBot "BigIDs" tests.  When
sizeof(T) is different than sizeof(dof_id_type) (as happened when an
8-byte-dof_id libMesh reads a file written with a 4-byte-dof_id
libMesh), we need to use the former rather than the latter to decide
what the code is for "no parent element".