objcopy doesn't handle ELF section groups

[philipc]

Untested, but it appears that sections with a type of SHT_GROUP will be parsed as SectionKind::Unknown, and written out with a sh_type set to SHT_PROGBITS.

Even with that fixed, we'll also need to translate the section contents, which contains an array of section indices.

Might be worth auditing objcopy behaviour for all unknown values. It should either read the value as some Unknown variant and then refuse to write it, or preserve the value unchanged.