Accept unordered certificate chains

Some TLS servers improperly send an unordered chain of certificates,
where the next certificate in the chain is not the issuer of the current
certificate. Recent versions of GnuTLS will verify the chain anyway to
help reduce unnecessary validation failures (since there is no security
risk in doing so).

When the certificates are unordered, get_peer_certificate_from_session()
will construct GTlsCertificates with incorrect issuer fields, causing
trouble with unordered chains even though GnuTLS would otherwise handle
these fine.

https://bugzilla.gnome.org/show_bug.cgi?id=683266