Add standard headers, C++ support to tls.h.

Add standard headers, C++ support to tls.h.

This makes using libtls easier to include by including dependent headers,
making something like this work as expected:

#include <iostream>
#include <tls.h>

int main()
{
   std::cout << "tls_init: " << tls_init() << "\n";
}

This also makes building a standalone libtls-portable simpler.

ok doug@, jsing@