(Imports upstream's 384e6de4c7. Changes to
P-256 assembly dropped because we're so different there.)

 - harmonize handlers with guidelines and themselves;
 - fix some bugs in handlers;

Change-Id: Ic0b6a37bed6baedc50448c72fab088327f12898d
Reviewed-on: https://boringssl-review.googlesource.com/13782
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>