MFC r308129:

When issuing a PXE dhcp request, always issue a param request (DHCP option 55)
with all dhcp parameters we might be interested in.

Some DHCP server like the new kea (by ISC) expect it.

This makes pxeboot functional with ISC kea.

Submitted by: Vincent Legout <vincent.legout@gandi.net>
Sponsored by: Gandi.net