Merge branch 'ikev2-signature-authentication'

This adds support for RFC 7427 signature authentication in IKEv2,
enabling the use of stronger signature schemes (e.g. RSA with SHA-2)
for IKE authentication.

Public key constraints defined in `rightauth` are now also checked
against IKEv2 signature schemes (may be disabled via strongswan.conf).

Fixes #863.