certstore: fix public dht call acceptance

Fix regression found in commit
[33447cc6: certstore: re-factor peer certificate authorization routine]

An unknown dht call is not accepted even if the configuration flags
"dhtPublicInCall" is true.

Now, accept the gnutls certificate error "INVALID + SIGNER_NOT_FOUND"
when this flags is true.

Change-Id: I90575ab82296d3d29140a7394ae178c196aa98c4
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>
Reviewed-by: Adrien Béraud <adrien.beraud@savoirfairelinux.com>

src/security/certstore.cpp

@@ -511,7 +511,8 @@ TrustStore::isAllowed(const crypto::Certificate& crt, bool allowPublic)
     // Match by certificate chain
     updateKnownCerts();
     auto ret = allowed_.verify(crt);
-    if (not ret) {
+    // Unknown issuer (only that) are accepted if allowPublic is true
+    if (not ret and !(allowPublic and ret.result == (GNUTLS_CERT_INVALID|GNUTLS_CERT_SIGNER_NOT_FOUND))) {
         RING_WARN("%s", ret.toString().c_str());
         return false;
     }