Make 'openssl req -x509' more equivalent to 'openssl req -new'

The following would fail, or rather, freeze:

    openssl genrsa -out rsa2048.pem 2048
    openssl req -x509 -key rsa2048.pem -keyform PEM -out cert.pem

In that case, the second command wants to read a certificate request
from stdin, because -x509 wasn't fully flagged as being for creating
something new.  This changes makes it fully flagged.

RT#4655

Reviewed-by: Andy Polyakov <appro@openssl.org>