Implement mutex and once-init synchronization primitives.  Wrap pthreads for now, but plan on raw futexes later.

Enable some useful Clang warnings, fix some bugs it detected.  Especially, eliminate all dynamic initializers.

Mark every destructor "noexcept(false)" unless it is very clearly noexcept.  The change in C++11 making destructors default-noexcept was a mistake.