rpki-client: fix and move more KU/EKU to x509_get_purpose()

rpki-client: fix and move more KU/EKU to x509_get_purpose()

Now all key usage and extended key usage handling is at the same place.
This fixes a bug for BGPsec Router certs where key usage was ignored.
Another omission that is fixed here is that criticality of the key usage
extension was not checked. Drop a comment about possible use of EKU that
was in the TA/CA code path but would only apply to EE certs.

ok claudio