dhcp_set_state() function

Create a DHCP request, fill in common headers

Set a statically allocated struct dhcp to work with. Using this prevents dhcp_start to allocate it using mem_malloc.

Extract the DHCP message and the DHCP options. Extract the DHCP message and the DHCP options, each into a contiguous piece of memory. As a DHCP message is variable sized by its options, and also allows overriding some fields for options, the easy approach is to first unfold the options into a contiguous piece of memory, and use that further on.