transport/{TCP,TLS}: optional IP_FREEBIND / IP_BINDANY bind socketops

Allows to bind to an address even if it is not actually (yet or ever)
configured. Fixes #238

Rationale:
https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/#whatdoesthismeanformeadeveloper

util/tcpsock/tcpsock_freebind_linux.go

@@ -0,0 +1,19 @@
+// +build linux
+
+package tcpsock
+
+import (
+	"syscall"
+)
+
+func freeBind(network, address string, c syscall.RawConn) error {
+	var err, sockerr error
+	err = c.Control(func(fd uintptr) {
+		// apparently, this works for both IPv4 and IPv6
+		sockerr = syscall.SetsockoptInt(int(fd), syscall.SOL_IP, syscall.IP_FREEBIND, 1)
+	})
+	if err != nil {
+		return err
+	}
+	return sockerr
+}