systemd/kresd.socket: explicitly set BindIPv6Only=both

This should be the default, but since we're relying on it in
our documentation, it's better to set it explicitly. It can be
overriden in the drop-in file if necessary.

distro/common/systemd/kresd-tls.socket

@@ -6,6 +6,7 @@ Before=sockets.target
 
 [Socket]
 FreeBind=true
+BindIPv6Only=both
 FileDescriptorName=tls
 ListenStream=[::1]:853
 ListenStream=127.0.0.1:853

distro/common/systemd/kresd.socket

@@ -6,6 +6,7 @@ Before=sockets.target
 
 [Socket]
 FreeBind=true
+BindIPv6Only=both
 ListenDatagram=[::1]:53
 ListenStream=[::1]:53
 ListenDatagram=127.0.0.1:53