Update to HDF5 1.8.16 with the HDFFV-9670 patch (btree2 code guard against corrupt HDF5 files) (merge from 14.12.x).

SVN: 35714

SVN: 32942

Add a hint to use variable-length strings for string compound members to modify the default mapping.

SVN: 32137

Enforce memory alignment for in-memory representations of HDF5 compounds (misalignments can lead to SIGBUS errors on non-Intel platforms).

SVN: 32135

SVN: 31688