[interface] Convert all data-xfer interfaces to generic interfaces

Remove data-xfer as an interface type, and replace data-xfer
interfaces with generic interfaces supporting the data-xfer methods.

Filter interfaces (as used by the TLS layer) are handled using the
generic pass-through interface capability.  A side-effect of this is
that deliver_raw() no longer exists as a data-xfer method.  (In
practice this doesn't lose any efficiency, since there are no
instances within the current codebase where xfer_deliver_raw() is used
to pass data to an interface supporting the deliver_raw() method.)

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

src/net/tcp/https.c

@@ -40,7 +40,7 @@ FEATURE ( FEATURE_PROTOCOL, "HTTPS", DHCP_EB_FEATURE_HTTPS, 1 );
  * @v uri		Uniform Resource Identifier
  * @ret rc		Return status code
  */
-static int https_open ( struct xfer_interface *xfer, struct uri *uri ) {
+static int https_open ( struct interface *xfer, struct uri *uri ) {
 	return http_open_filter ( xfer, uri, HTTPS_PORT, add_tls );
 }