Fix configuration of ASN1_DN IDs.

Fix configuration of ASN1_DN IDs.

Public key authentication uses public key files that are stored in the
/etc/iked/pubkeys/ directory where the IKE IDs are encoded as filenames.
This does not simply work with ASN1_DNs where the IDs include slashes
and other special characters. Instead of breaking and failing when an
ASN1_DN is configured, simply skip the public key lookup but allow
to use it with certificates or PSKs.

Reported and fix tested by Igor V. Gubenko - Thanks.