nixos/modules/system/boot/networkd: enable socket activation

Since cd1dedac systemd-networkd has it's
netlink socket created via a systemd.socket unit. One might think that
this doesn't make much sense since networkd is just going to create it's
own socket on startup anyway. The difference here is that we have
configuration-time control over things like socket buffer sizes vs
compile-time constants.

For larger setups where networkd has to create a lot of (virtual)
devices the default buffer size of currently 128MB is not enough.

A good example is a machine with >100 virtual interfaces (e.g.,
wireguard tunnels, VLANs, …) that all have to be brought up during
startup. The receive buffer size will spike due to all the generated
message from the new interfaces. Eventually some of the message will be
dropped since there is not enough (permitted) buffer space available.

By having networkd start through / with a netlink socket created by
systemd we can configure the `ReceiveBufferSize` parameter in the socket
options without recompiling networkd.

Since the actual memory requirements depend on hardware, timing, exact
configurations etc. it isn't currently possible to infer a good default
from within the NixOS module system. Administrators are advised to
monitor the logs of systemd-networkd for `rtnl: kernel receive buffer
overrun` spam and increase the memory as required.

Note: Increasing the ReceiveBufferSize doesn't allocate any memory.  It
just increases the upper bound on the kernel side. The memory allocation
depends on the amount of messages that are queued on the kernel side of
the netlink socket.