Add 1.6.4 notes to the NEWS file
Very minor update: leave MS Windows in the "lightly tested" catetory,
but remove it from the "fully tested" category :-(
Bump up to 1.6.4rc1
Update shared library versions for upcoming 1.6.4 release.
Add note about the inadvertant 1.6.3 "use mpi" ABI breakage.