archive/zip: FileHeader.Extra API is problematic

[dsnet]

The FileHeader.Extra field is used by the Writer to write the "extra" field for the local file header and the central-directory file header. This is problematic because the Go implementation assumes that the extra bytes used in the two headers are the same. While is this is often the case, it is not always true.

See http://mdfs.net/Docs/Comp/Archiving/Zip/ExtraField and you will notice that it frequently describes a "Local-header version" and a "Central-header version", where the formats sometimes differ.

The Reader does not have this problem because it entirely ignores the local headers.

I haven't thought much about what the right action is moving forward, whether to deprecate this field or add new API. I just want to file this issue, so I remember to address it later.

[ianlancetaylor]

@dsnet This is currently set for the 1.11 milestone. Please update as appropriate. Thanks.

[rsc]

I suggest we add ExtraCentral []byte, where a writer treats nil to mean "same as Extra".