systemd_networkd(role),router(host): support global config to fix forw.

With the router upgrade to Debian 13 the systemd version got upgraded as
well breaking the current configuration for IP forwarding.
Add a variable for global systemd-networkd configuration and use that to
enable IPv4 and IPv6 forwarding on the router.

The systemd_networkd role could be a bit nicer, not deploying/deleting
the global configuration, if the variable is empty and
reloading/restarting systemd-networkd at appropriate times. But as is
works for now.

resources/chaosknoten/router/systemd_networkd/20-net1.network

@@ -3,7 +3,6 @@ Name=net1
 
 [Network]
 DNS=212.12.50.158
-IPForward=ipv4
 IPv6AcceptRA=no
 
 [Address]
@@ -11,4 +10,3 @@ Address=212.12.48.123/24
 
 [Route]
 Gateway=212.12.48.55
-

resources/chaosknoten/router/systemd_networkd/20-net2.network

@@ -3,7 +3,6 @@ Name=net2
 
 [Network]
 #DNS=212.12.50.158
-IPForward=ipv6
 IPv6AcceptRA=no
 
 [Address]
@@ -11,4 +10,3 @@ Address=2a00:14b0:4200:3500::130:2/112
 
 [Route]
 Gateway=2a00:14b0:4200:3500::130:1
-

resources/chaosknoten/router/systemd_networkd_global_config.conf

@@ -0,0 +1,3 @@
+[Network]
+IPv4Forwarding=true
+IPv6Forwarding=true