Fix all warnings in the main codebase flagged by -Wsigned-compare

Fix all warnings in the main codebase flagged by -Wsigned-compare

Remember, the code
   int is_less_than(int a, unsigned b) {
      return a < b;
   }
is buggy, since the C integer promotion rules basically turn it into
   int is_less_than(int a, unsigned b) {
      return ((unsigned)a) < b;
   }
and we really want something closer to
   int is_less_than(int a, unsigned b) {
      return a < 0 || ((unsigned)a) < b;
   }
.

Suggested by an example from Ralph Castain