Add wait-network-online.service

Add a service which waits (max 15 seconds) for the network to be
available. We define 'available' as the ability to ping our default
router by its name, which implies dns is working.

Service units, which really need network to start, should add

    After=network-online.target
    Wants=network-online.target

install.sh

@@ -264,6 +264,8 @@ install_symlink /etc/local/access.conf                    "$DESTDIR$sysconfdir/s
 install_data misc_etc_files/cups/client.conf              "$DESTDIR$sysconfdir/cups/client.conf"
 install_data misc_systemd_units/ip6.service               "$DESTDIR$systemdunitdir/ip6.service"
 install_data mxrouter/mxrouter.service                    "$DESTDIR$systemdunitdir/mxrouter.service"
+install_data misc_systemd_units/wait-network-online.service \
+                                                          "$DESTDIR$systemdunitdir/wait-network-online.service"
 
 postinstall
 exit

misc_systemd_units/wait-network-online.service

@@ -0,0 +1,14 @@
+[Unit]
+DefaultDependencies=no
+After=sysinit.target network.target
+Before=network-online.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=true
+TimeoutStartSec=15
+StandardOutput=null
+ExecStart=bash -c 'while ! ping -c 1 -w 1 rick.molgen.mpg.de; do sleep 1 ; done'
+
+[Install]
+WantedBy=network-online.target