ipv4: add Header.ParseWire and Header.MarshalWire

[denisvmedia]

Header.Parse and Header.Marshal use the format of a raw IP socket on
the local system. On Darwin, iOS, DragonFly BSD, NetBSD and older
FreeBSD the TotalLen and FragOff fields are in host byte order there,
and TotalLen may exclude the header length. A header captured on Linux,
which is in the wire format, is therefore parsed incorrectly on those
systems, and there is no way to parse it portably.

Add ParseWire and MarshalWire, which always use the wire format defined
in RFC 791. Describe the per-system differences with an internal
rawFormat type so that Parse/Marshal and the new methods share one
implementation. The behavior of Parse, Marshal and ParseHeader does not
change.

Fixes golang/go#43386

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[denisvmedia]

(don't won't to include my email in a publicly visible commit for CLA, but anyone can use this code freely)