networkd: propagate IPFoward= per-interface setting also to /proc/sys/net/ipv4/ip_forward

We need to turn on /proc/sys/net/ipv4/ip_forward before the
per-interface forwarding setting is useful, hence let's propagate the
per-interface setting once to the system-wide setting.

Due to the unclear ownership rules of that flag, and the fact that
turning it on also has effects on other sysctl flags we try to minimize
changes to the flag, and only turn it on once. There's no logic to
turning it off again, but this should be fairly unproblematic as the
per-interface setting defaults to off anyway.