wireguard: netlink: add YNL specification

wireguard: netlink: add YNL specification

This patch adds a near[1] complete YNL specification for WireGuard,
documenting the protocol in a machine-readable format, rather than
comments in wireguard.h, and eases usage from C and non-C programming
languages alike.

The generated C library will be featured in a later patch, so in
this patch I will use the in-kernel python client for examples.

This makes the documentation in the UAPI header redundant, it is
therefore removed. The in-line documentation in the spec is based
on the existing comment in wireguard.h, and once released it will
be available in the kernel documentation at:
  https://docs.kernel.org/netlink/specs/wireguard.html
  (until then run: make htmldocs)

Generate wireguard.rst from this spec:
$ make -C tools/net/ynl/generated/ wireguard.rst

Query wireguard interface through pyynl:
$ sudo ./tools/net/ynl/pyynl/cli.py --family wireguard \
                                    --dump get-device \
                                    --json '{"ifindex":3}'
[{'fwmark': 0,
  'ifindex': 3,
  'ifname': 'wg-test',
  'listen-port': 54318,
  'peers': [{0: {'allowedips': [{0: {'cidr-mask': 0,
                                     'family': 2,
                                     'ipaddr': '0.0.0.0'}},
                                {0: {'cidr-mask': 0,
                                     'family': 10,
                                     'ipaddr': '::'}}],
                 'endpoint': b'[...]',
                 'last-handshake-time': {'nsec': 42, 'sec': 42},
                 'persistent-keepalive-interval': 42,
                 'preshared-key': '[...]',
                 'protocol-version': 1,
                 'public-key': '[...]',
                 'rx-bytes': 42,
                 'tx-bytes': 42}}],
  'private-key': '[...]',
  'public-key': '[...]'}]

Add another allowed IP prefix:
$ sudo ./tools/net/ynl/pyynl/cli.py --family wireguard \
  --do set-device --json '{"ifindex":3,"peers":[
    {"public-key":"6a df b1 83 a4 ..","allowedips":[
      {"cidr-mask":0,"family":10,"ipaddr":"::"}]}]}'

[1] As can be seen above, the "endpoint" is only dumped as binary data,
    as it can't be fully described in YNL. It's either a struct
    sockaddr_in or struct sockaddr_in6 depending on the attribute length.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>