Reintroduce 5.3-style checksum modification to preserve end-to-end checksums

Reintroduce 5.3-style checksum modification to preserve end-to-end checksums
when fiddling with packets but without the mess that motivated Henning to
remove it. Affects only this one aspect of Henning's checksum work. Also tweak
the basic algorithm and supply a correctness argument.

OK dlg@ deraadt@ sthen@; no objection henning@