The callback arguments are required to be NULL.

Change-Id: I266ec46efdaca411a7f0c2b645883b2c5bec1c96
Reviewed-on: https://boringssl-review.googlesource.com/5160
Reviewed-by: Adam Langley <agl@google.com>

Recognize the four most commonly offered safe DH parameter sets when
negotiating multiplicative, ephemeral Diffie-Hellman. These values were
found from a scan of the Alexa common sites.

When a known safe prime is used, reduce the private key size
correspondingly.

Change-Id: I655eb7a5c743c0b389698c0471d16db5a0966652

Initial fork from f2d678e6e8 (1.0.2 beta).

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