[dhcp] Handle missing ProxyDHCP servers
It is possible to configure a DHCP server to hand out PXE options
without a ProxyDHCP server present. This requires setting option 60
to "PXEClient", which will cause gPXE to attempt ProxyDHCP.
We assume in several places that dhcp->proxydhcpack is set to the
DHCPACK packet containing option 60 set to "PXEClient". When we
transition into ProxyDHCPREQUEST, set dhcp->proxydhcpack=dhcp->dhcpack
so that this assumption holds true.
We ought to rename several references to "proxydhcp" to something more
accurate, such as "pxedhcp". Treating a single DHCP response as
potentially both DHCPOFFER and ProxyDHCPOFFER does make the code
smaller, but the variable names get confusing.