Change many tortls.h declarations of private APIs to use structs

This change makes it so those those APIs will not require prior
inclusion of openssl headers.  I've left some APIs alone-- those
will change to be extra-private.

src/common/tortls.c

@@ -17,6 +17,7 @@
 #include "orconfig.h"
 
 #define TORTLS_PRIVATE
+#define TORTLS_OPENSSL_PRIVATE
 
 #include <assert.h>
 #ifdef _WIN32 /*wrkard for dtls1.h >= 0.9.8m of "#include <winsock.h>"*/