dhclient: improve UDP checksum handling

When sending UDP packets:

• compute the checksum in the correct order. This only has an impact if the length of the payload is odd.
• don't send packet with a checksum of zero, use 0xffff instead as required.

When receiving UDP packets:

• don't do any computations when the checksum is zero.
• compute the checksum in the correct order. This only has an impact if the length of the payload is odd.
• when computing the checksum, store the pseudo header checksum
• if the checksum is computed as zero, use 0xffff instead.
• also accept packets, when the checksum in the packet is the pseudo header checksum.

The last point fixes a problem when the DHCP client runs in a VM,
the DHCP server runs on the host serving the VM and the network
interface supports transmit checksum offloading. Since dhclient
doesn't use UDP sockets but bpf devices to read the packets, the
checksum will be incorrect and only contain the checksum of the
pseudo header.

PR: 263229
Reviewed by: markj, Timo Völker
Tested by: danilo
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52394