Add support for new ipxlat kernel driver (WIP)

Needs the kernel module from
https://codeberg.org/IPv6-Monostack/ipxlat-net-next. This can be built
and loaded out-of-tree like so (assuming you've got all the necessary
kernel development packages (make, gcc, kernel-headers etc.) installed:

```
git clone --depth 1 https://codeberg.org/IPv6-Monostack/ipxlat-net-next
cd ipxlat-net-next/drivers/net/ipxlat
make -f /lib/modules/$(uname -r)/build/Makefile M=$PWD CONFIG_IPXLAT=m CFLAGS_MODULE=-I$PWD/../../../include modules
sudo make -f /lib/modules/$(uname -r)/build/Makefile M=$PWD CONFIG_IPXLAT=m CFLAGS_MODULE=-I$PWD/../../../include modules_install
sudo modprobe ipxlat
```

You'll also need to install the 'ipxlat-ctl' utility from
https://codeberg.org/IPv6-Monostack/ipxlat somewhere in your $PATH (or
point clatd's 'cmd-ipxlat-ctl' config option at its location).

(Note: I was not able to make the above 'ipxlat-ctl' utility work, so I
have made an alternative 'ipxlat-ctl' script located in the root of the
clatd repo. You probably need to edit that script so that it can find
the pyynl cli utility included in the kernel sources.)

For now, only /96 RFC6052 prefixes are supported. Support for the other
prefix lengths will be added later.

ipxlat-ctl

@@ -0,0 +1,18 @@
+#!/bin/bash -e
+
+# This is a wrapper around the python ynl cli included with the kernel that
+# mimics the documented behaviour of the ipxlat-ctl tool available at
+# https://codeberg.org/IPv6-Monostack/ipxlat, but which I haven't been able to
+# make work. You'll need to ensure the tool is somewhere in $PATH.
+
+PATH=/usr/src/kernels/ipxlat-net-next/tools/net/ynl/pyynl:$PATH
+
+dev="$1"
+prefix="$3"
+
+IID=$(< /sys/class/net/$dev/ifindex)
+ADDR_HEX=$(python3 -c 'import ipaddress,sys; print(ipaddress.IPv6Address(sys.argv[1]).packed.hex())' ${prefix%/*})
+PREFIXLEN=${prefix#*/}
+JSON='{"ifindex": '"$IID"', "config": {"xlat-prefix6": { "prefix": "'$ADDR_HEX'", "prefix-len": '$PREFIXLEN'}}}'
+
+cli.py --family ipxlat --do dev-set --json "$JSON"