Add SSL_early_get1_extensions_present()

It is an API to be used from the early callback that indicates what
extensions were present in the ClientHello, and in what order.
This can be used to eliminate unneeded calls to SSL_early_get0_ext()
(which itself scales linearly in the number of extensions supported
by the library).

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2976)