Fix *_ctx_new and *_ctx_free implementations

They both will only alloc and free the internal *_ctx structs. Get rid
of the void * argument for new and only pass arg to *_free instead of
the whole lc_*_ctx struct.

cipher_chacha20.c

@@ -179,7 +179,7 @@ chacha20_x(const uint8_t *key, size_t keylen, const uint8_t *iv, size_t ivlen,
 }
 
 static void *
-chacha20_ctx_new(const void *arg)
+chacha20_ctx_new(void)
 {
 	return malloc(sizeof(struct chacha20_ctx));
 }