botan: Use strongSwan's RNG interface in Botan plugin

This allows using rng_t implementations provided by other plugins to
serve as RNG for Botan.

Closes strongswan/strongswan#192.

src/libstrongswan/plugins/botan/botan_rsa_private_key.c

@@ -369,7 +369,7 @@ botan_rsa_private_key_t *botan_rsa_private_key_gen(key_type_t type,
 		return NULL;
 	}
 
-	if (botan_rng_init(&rng, "system"))
+	if (!botan_get_rng(&rng, RNG_TRUE))
 	{
 		return NULL;
 	}
@@ -448,7 +448,7 @@ static bool calculate_pq(botan_mp_t *n, botan_mp_t *e, botan_mp_t *d,
 		goto error;
 	}
 
-	if (botan_rng_init(&rng, "user"))
+	if (!botan_get_rng(&rng, RNG_STRONG))
 	{
 		goto error;
 	}