Author: Thomas Sondergaard <tsondergaard@vitalimages.com>
Date:   Sat Jun 16 18:14:50 2018 +0200

   Eliminate use of the 'register' keyword

   'register' is a reserved and unused keyword in C++17 so having it
   present in the public headers presents a a compatibility issue. Also
   in C the 'register' keyword is mostly obsolete, so remove all uses of
   it.

   [ghudson@mit.edu: adjusted style of some of the affected lines]