[xfer] Add xfer_window_changed()

[xfer] Add xfer_window_changed()

xfer_window_changed() can be used to notify peers that an interface is
now ready to accept data.  This can potentially be used to eliminate
the need for wasteful processes that simply poll xfer_window() until
the window becomes non-zero.

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