Fix some issues in apps/req

1. the 'ignore -days' warning should not be printed without '-x509'
2. the 'ignore -days' warning should terminate with new-line

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4767)