Clarify/fix encoder/decoder context docs and code

Clarify/fix encoder/decoder context docs and code

In was premature to make OSSL_(EN|DE)CODER_CTX_[sg]et_finalized() be
public interfaces.  Forunately, these have not yet appeared outside the
"master" branch, so we can still retract them.

Also, in the case of decoders, the implementation failed to take into
account that the context was duplicated before it was returned to the
user, and the duplicated copy failed to copy the "finalized" field.

This commit also renames "finalized" to "frozen", because
finalisation is a misleading term in this context, it suggests
resource reclamation during garbage collection or deallocation,
not marking a structure partly immutable.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/29206)