There are a few test vectors which were not imported from djb's. Mirror
those. Also as RFC 8032 uses a slightly different private key
representation, document this in curve25519.h.
BUG=187
Change-Id: I119381168ba1af9b332365fd8f974fba41759d57
Reviewed-on: https://boringssl-review.googlesource.com/14445
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>