Adapt int_ctx_new() to use with providers

Adapt int_ctx_new() to use with providers

This affects all its callers: EVP_PKEY_CTX_new(), EVP_PKEY_CTX_new_id().
They are now possible to called with "zero" values, i.e.:

    EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new(NULL, NULL);

or

    EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(0, NULL);

This is suitable for provider use, as the key functionality is tied
with its keys, and the operation time is determined by the init
functions the EVP_PKEY_CTX is used with.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9312)