Allow for differentiating between default and NULL context

Allow for differentiating between default and NULL context

Allow for use of default context in the thread-local api by using
get_concrete

This creates a problem however in that get_concrete access a
thread-local api, creating a recursive call that exhausts stack space

So create a special context token that identifys the no context use case
when we want to store thread-local data without an explicit context
index

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27794)