[interface] Allow for non-pass-through interface methods

xfer_vredirect() should not be allowed to propagate to a pass-through
interface.  For example, when an HTTPS connection is opened, the
redirect message should cause the TLS layer to reopen the TCP socket,
rather than causing the HTTP layer to disconnect from the TLS layer.

Fix by allowing for non-pass-through interface methods, and setting
xfer_vredirect() to be one such method.

This is slightly ugly, in that it complicates the notion of an
interface method call by adding a "pass-through" / "non-pass-through"
piece of metadata.  However, the only current user of xfer_vredirect()
is iscsi.c, which uses it only because we don't yet have an
ioctl()-style call for retrieving the underlying socket address.
The new interface infrastructure allows for such a call to be created,
at which time this sole user of xfer_vredirect() can be removed,
xfer_vredirect() can cease to be an interface method and become simply
a wrapper around xfer_vreopen(), and the concept of a non-pass-through
interface method can be reverted.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/xfer.c

@@ -56,7 +56,7 @@ static struct xfer_metadata dummy_metadata;
 int xfer_vredirect ( struct interface *intf, int type, va_list args ) {
 	struct interface *dest;
 	xfer_vredirect_TYPE ( void * ) *op =
-		intf_get_dest_op ( intf, xfer_vredirect, &dest );
+		intf_get_dest_op_no_passthru ( intf, xfer_vredirect, &dest );
 	void *object = intf_object ( dest );
 	int rc;