HeadSpin.h cleanup

Clean up some unnecessary and unused stuff, mostly from HeadSpin.h.  Switch to STL versions of common routines (e.g. ```min``` and ```max```) where applicable.
That would break more things than it would fix...  The Windows headers use macros, which means that any code that has 'min' or 'max' names get replaced by the macro contents, and std::<some expression>(val1, val2) doesn't work well.