index: avoid indexing legacy-display parts

When we notice a legacy-display part during indexing, it makes more
sense to avoid indexing it as part of the message body.

Given that the protected subject will already be indexed, there is no
need to index this part at all, so we skip over it.

If this happens during indexing, we set a property on the message:
index.repaired=skip-protected-headers-legacy-display

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>