diffie-hellman: Explicitly initialize DH exponent sizes during initialization

To avoid any race conditions when multiple threads call and initialize
diffie_hellman_get_params(), explicitly examine the optimum DH exponent size
during library initialization.

Fixes #655.

src/libstrongswan/crypto/diffie_hellman.h

@@ -148,9 +148,17 @@ struct diffie_hellman_params_t {
 	const chunk_t subgroup;
 };
 
+/**
+ * Initialize diffie hellman parameters during startup.
+ */
+void diffie_hellman_init();
+
 /**
  * Get the parameters associated with the specified diffie hellman group.
  *
+ * Before calling this method, use diffie_hellman_init() to initialize the
+ * DH group table. This is usually done by library_init().
+ *
  * @param group			DH group
  * @return				The parameters or NULL, if the group is not supported
  */