Fix #946 Add -preserve_dates to x509 app

Add the -preserve_dates dates option to preserve dates when signing
a certificate.
Prevent -days and -preserve_dates being used simultaneously
Fixes #946

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/946)