Some end-user configurations have been observed in which the first NBP
(such as GRUB2) uses the UNDI API and then transfers control to a
second NBP (such as pxelinux) which uses the UDP API.  The first NBP
closes the network device using PXENV_UNDI_CLOSE, which renders the
UDP API unable to transmit or receive packets.

The correct behaviour under these circumstances is (as often) simply
not documented by the PXE specification.  Testing with the Intel PXE
stack suggests that PXENV_UDP_OPEN will implicitly reopen the network
device if necessary, so match this behaviour.

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