[dhcp] Omit ProxyDHCPREQUEST if PXE options are present in ProxyDHCPOFFER

[dhcp] Omit ProxyDHCPREQUEST if PXE options are present in ProxyDHCPOFFER

Some ProxyDHCP implementations seem to violate the PXE specification
by expecting the client to retain options from the ProxyDHCPOFFER
rather than issuing a separate ProxyDHCPREQUEST.

Work around such broken clients by retaining the ProxyDHCPOFFER
packet, and proceeding to a ProxyDHCPREQUEST only if the
ProxyDHCPOFFER does not already contain PXE options.

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