systemd: only override issues with stock units

Only oidentd.socket and miredo.service were copied instead of being
units that exist in the system and they don't need to do anything
else than fix the issue I have with the stock units.

* oidentd.socket is IPv6-only on my systems unless is BindIPv6Only=both.
  because of net.ipv6.bindv6only=1
* miredo.service is here because it starts before there is network
  connection (network-online.target) and there is never network
  connection with laptops before they are connected to WLAN even if
  NetworkManager might be up seeking/connecting to network.

etc/systemd/system/README.md

@@ -1,6 +1,6 @@
 Systemd services. These are sorted by some kind of category into
 subdirectories. The sudirectories won't exist in the real
-`/etc/systemd/system` unless they end `.wants` or something similar
+`/etc/systemd/system` unless they end `.wants` or `.d` or something similar
 and I forget to update this README file if that happens.
 
 * reflector.service is copied from https://wiki.archlinux.org/index.php/Reflector

etc/systemd/system/ipv6/miredo.service

@@ -1,10 +0,0 @@
-[Unit]
-Description=Teredo IPv6 Tunneling Daemon
-After=network-online.target unbound.service
-
-[Service]
-ExecStart=/usr/bin/miredo -f
-ExecReload=/bin/kill -HUP $MAINPID
-
-[Install]
-WantedBy=multi-user.target

etc/systemd/system/miredo.service.d/override.conf

@@ -0,0 +1,3 @@
+[Unit]
+# If unbound is used for DNS, also add unbound.service
+After=network-online.target

etc/systemd/system/oidentd.socket

@@ -1,10 +0,0 @@
-[Unit]
-Description=Ident (RFC 1413) socket
-
-[Socket]
-ListenStream=113
-BindIPv6Only=both
-Accept=yes
-
-[Install]
-WantedBy=sockets.target

etc/systemd/system/oidentd.socket.d/override.conf

@@ -0,0 +1,2 @@
+[Socket]
+BindIPv6Only=both