wg: ipc: add allowed-ip flags support for FreeBSD

For $reasons, we can't rely on flags in wireguard-tools for the kernel
side of WireGuard.  Provide a mapping function that uses flags from the
kernel that we're building against and fail the operation if we made it
to the end without turning some wg(8) flag off.

Signed-off-by: Kyle Evans <kevans@FreeBSD.org>

Reviewed by:	ivy (previous version), Jason A. Donenfeld, jhb
Differential Revision:	https://reviews.freebsd.org/D50450

contrib/wireguard-tools/ipc-freebsd.h

@@ -307,11 +307,8 @@ static int kernel_set_device(struct wgdevice *dev)
 			nvl_aips[j] = nvlist_create(0);
 			if (!nvl_aips[j])
 				goto err_peer;
-			if (aip->flags) {
-				//TODO: implement me
-				ret = -EOPNOTSUPP;
-				goto err_peer;
-			}
+			if (aip->flags)
+				nvlist_add_number(nvl_aips[j], "flags", aip->flags);
 			nvlist_add_number(nvl_aips[j], "cidr", aip->cidr);
 			if (aip->family == AF_INET)
 				nvlist_add_binary(nvl_aips[j], "ipv4", &aip->ip4, sizeof(aip->ip4));