lib/connections: Handle QUIC not being available (#7186)

This does two things:

- Exclude QUIC from go1.16 builds, automatically, for now, since it
  doesn't work and just panics.

- Provide some fake listeners and dialers when QUIC is disabled.

These fake listeners and dialers indicate that they are disabled and
unsupported, which silences "Dialing $address: unknown address scheme:
quic" type of stuff which is not super helpful to the user.

lib/connections/quic_unsupported.go

@@ -0,0 +1,16 @@
+// Copyright (C) 2020 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// +build noquic !go1.14 go1.16
+
+package connections
+
+func init() {
+	for _, scheme := range []string{"quic", "quic4", "quic6"} {
+		listeners[scheme] = invalidListener{err: errNotInBuild}
+		dialers[scheme] = invalidDialer{err: errNotInBuild}
+	}
+}