DHCP: Fix checksum validation

DHCP: Fix checksum validation

in_cksum falls over with struct ip in a union of uint16_t with
some compilers.

The fix is to create a smaller pseudo header, fill in the bits
we need and then copy this to a uint8_t array which we then
send to in_cksum.

Tested on Debian-12 with clang-15 and CFLAGS=-Os

While here, just check that the UDP checksum check is zero
rather than zeroing it out and checking it matches.

Co-authored-by: Zikai Chen <chenzikai@google.com>