Backport X509_check_private_key.pod

to address #3973, and original PR to master branch is #3614

test case in the original PR is not applied.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4002)