Move the registration of callback functions to special functions

designed for that.  This removes the potential error to mix data and
function pointers.

Please note that I'm a little unsure how incorrect calls to the old
ctrl functions should be handled, in som cases.  I currently return 0
and that's it, but it may be more correct to generate a genuine error
in those cases.

crypto/bio/bss_log.c

@@ -100,6 +100,7 @@ static BIO_METHOD methods_slg=
 	slg_ctrl,
 	slg_new,
 	slg_free,
+	NULL,
 	};
 
 BIO_METHOD *BIO_s_log(void)