Also, restore a behaviour change, where load_cert() would look at
stdin when the input file name is NULL, and make sure to call
load_cert_pass() with a corresponding argument where load_cert() was
used in OpenSSL 1.1.1.

Fixes #13235
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/13236)