This allows us to fix another consumer that directly accesses SSL_CTX.
I've made ssl_test use it for test coverage, though we're okay with
ssl_test depending on ssl/internal.h.

Bug: 6
Change-Id: I464325e3faa9f0194bbd357fbb31a996afc0c2e1
Reviewed-on: https://boringssl-review.googlesource.com/18964
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>