pki: Make --dn optional for certificate renewals via --scep command

pki: Make --dn optional for certificate renewals via --scep command

When using OpenXPKI, the subject DN in the renewal request has to match
the previous DN exactly.  However, because OpenXPKI may add a bunch of
DC/O RDNs to subjects of issued certificates, running --scep with the
same --dn that was used for the original request won't work (results in
a "Client error / malformed request badRequest" error even after enabling
`renewal_via_pkcs_req`).  This simplifies renewals as --dn can just be
omitted and extracted from the original certificate to avoid this issue.

References strongswan/strongswan#1689