pkg: listeners: separate out the listeners package

This code will be used in containerd and is quite useful in general to
people who want a nice way of creating listeners from proto://address
arguments (even supporting socket activation). Separate it out from
docker/ so people can use it much more easily.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

listeners/listeners.go

@@ -0,0 +1,22 @@
+package listeners
+
+import (
+	"crypto/tls"
+	"net"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/docker/go-connections/sockets"
+)
+
+func initTCPSocket(addr string, tlsConfig *tls.Config) (l net.Listener, err error) {
+	if tlsConfig == nil || tlsConfig.ClientAuth != tls.RequireAndVerifyClientCert {
+		logrus.Warn("/!\\ DON'T BIND ON ANY IP ADDRESS WITHOUT setting -tlsverify IF YOU DON'T KNOW WHAT YOU'RE DOING /!\\")
+	}
+	if l, err = sockets.NewTCPSocket(addr, tlsConfig); err != nil {
+		return nil, err
+	}
+	if err := allocateDaemonPort(addr); err != nil {
+		return nil, err
+	}
+	return
+}