Imported from upstream's 918bb86529. (The
remainder is in code we've since deleted.)

Change-Id: Ie878272114086ba60a0fd5eae059b641b00ec0c4
Reviewed-on: https://boringssl-review.googlesource.com/3831
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)