Unlike the Scoped* types, bssl::UniquePtr is available to C++ users, and
offered for a large variety of types.  The 'extern "C++"' trick is used
to make the C++ bits digestible to C callers that wrap header files in
'extern "C"'.

Change-Id: Ifbca4c2997d6628e33028c7d7620c72aff0f862e
Reviewed-on: https://boringssl-review.googlesource.com/10521
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>