Fixes #3620: Move DDT fix to v1.6

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r28568 [[BR]]
Based on the MPI 3.0 count equal to zero has a clear meaning, no modification
of the original datatype are allowed (not in type map nor extent). Make it
clear in the code.
Allow 0-count cases to the contiguous memory check.
Update news for 1.6.5
Update shared library version numbers for the upcoming 1.6.5 release.

 * we changed the contents of a static inline function in
   opal/datatype/opal_datatype.h, which also changed:
   * libmpi_f77
   * libmpi_cxx
   * libmpi
   * libmca_common_sm
 * we also changed libmpi independently of this

All other changes were in components or non-code files.  Hence, all of
these libraries got a rev bump.
On to 1.6.5rc1