openssl rehash: use libcrypto variables for default dir

X509_get_default_cert_dir_env() returns the default environment
variable to check for certificate directories.
X509_get_default_cert_dir() returns the default configured certificate
directory.

Use these instead of hard coding our own values, and thereby be more
integrated with the rest of OpenSSL.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5937)