Make it possible to generate proxy certs with test/certs/mkcert.sh

This extends 'req' to take more than one DN component, and to take
them as full DN components and not just CN values.  All other commands
are changed to pass "CN = $cn" instead of just a CN value.

This adds 'genpc', which differs from the other 'gen*' commands by not
calling 'req', and expect the result from 'req' to come through stdin.

Finally, test/certs/setup.sh gets the commands needed to generate a
few proxy certificates.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Stephen Henson <steve@openssl.org>