Always set SSL_OP_SINGLE_DH_USE.

This is an API wart that makes it easy to accidentally reuse the server
DHE half for every handshake. It's much simpler to have only one mode.
This mirrors the change made to the ECDHE code; align with that logic.

Change-Id: I47cccbb354d70127ab458f99a6d390b213e4e515
Reviewed-on: https://boringssl-review.googlesource.com/4565
Reviewed-by: Adam Langley <agl@google.com>
3 files changed