Move GeneralName parsing code to ipalib.x509
GeneralName parsing code is primarily relevant to X.509. An
upcoming change will add SAN parsing to the cert-show command, so
first move the GeneralName parsing code from ipalib.pkcs10 to
ipalib.x509.
Part of: https://fedorahosted.org/freeipa/ticket/6022
Reviewed-By: Jan Cholasta <jcholast@redhat.com>