nixos/knot: add support for XDP setups

The Express Data Path (XDP) is a way to circumvent the traditional Linux
networking stack and instead run an eBPF program on your NIC, that makes
the decision to provide Knot with certain packets. This is way faster
and more scalable but comes at the cost of reduced introspection.

Unfortunately the `knotc conf-check` command fails hard with missing
interfaces or IP addresses configured in `xdp.listen`, so we disable it
for now, once the `xdp` config section is set. We also promote the config
check condition to a proper option, so our conditions become public
documentation, and we allow users to deal with corner cases, that we have
not thought of yet.

We follow the pre-requisites documented in the Knot 3.3 manual, and set
up the required capabilities and allow the AF_XDP address family.

But on top of that, due to our strict hardening, we found two more
requirements, that were communicated upstream while debugging this.

- There is a requirement on AF_NETLINK, likely to query for and configure
  the relevant network interface
- Running eBPF programs requires access to the `bpf` syscall, which we
  deny through the `~@privileged` configuration.

In summary We now conditionally loosen the hardening of the unit once we
detect that an XDP configuration is wanted. And since we cannot
introspect arbitrary files from the `settingsFiles` option, we expose XDP
support through the `enableXDP` toggle option on the module.

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