Use an unsigned type (size_t) for all loop iterators.

Use an unsigned type (size_t) for all loop iterators.

This resolves a warning from gcc 4.5:
  assuming signed overflow does not occur when simplifying
    conditional to constant

This page explains the issues in some detail:
  http://www.airs.com/blog/archives/120
(cherry picked from commit 321ca1ef91a90cec5b94058b84420e8018e3f1d8)