Fix by dhartmei@ and mcbride@
1.433
Properly m_copyback() modified TCP sequence number after demodulation
1.432
Fix icmp checksum when sequence number modlation is being used.
Also fix a daddr vs saddr cut-n-paste error in ICMP error handling.
Fixes PR 3724
Obtained from: OpenBSD
Reviewed by: dhartmei
Approved by: rwatson