Although we are derived from 1.0.2, we mimic 1.1.0 in some ways around
our FOO_up_ref functions and opaque libssl types. This causes some
difficulties when porting third-party code as any OPENSSL_VERSION_NUMBER
checks for 1.1.0 APIs we have will be wrong.

Moreover, adding accessors without changing OPENSSL_VERSION_NUMBER can
break external projects. It is common to implement a compatibility
version of an accessor under #ifdef as a static function. This then
conflicts with our headers if we, unlike OpenSSL 1.0.2, have this
function.

This change switches OPENSSL_VERSION_NUMBER to 1.1.0 and atomically adds
enough accessors for software with 1.1.0 support already. The hope is
this will unblock hiding SSL_CTX and SSL_SESSION, which will be
especially useful with C++-ficiation. The cost is we will hit some
growing pains as more 1.1.0 consumers enter the ecosystem and we
converge on the right set of APIs to import from upstream.

It does not remove any 1.0.2 APIs, so we will not require that all
projects support 1.1.0. The exception is APIs which changed in 1.1.0 but
did not change the function signature. Those are breaking changes.
Specifically:

- SSL_CTX_sess_set_get_cb is now const-correct.

- X509_get0_signature is now const-correct.

For C++ consumers only, this change temporarily includes an overload
hack for SSL_CTX_sess_set_get_cb that keeps the old callback working.
This is a workaround for Node not yet supporting OpenSSL 1.1.0.

The version number is set at (the as yet unreleased) 1.1.0g to denote
that this change includes https://github.com/openssl/openssl/pull/4384.

Bug: 91
Change-Id: I5eeb27448a6db4c25c244afac37f9604d9608a76
Reviewed-on: https://boringssl-review.googlesource.com/10340
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>