netlink: group rt, genl and channel under a netlink namespace

[lneto]

Constructing a netlink session read as an anonymous call on the required class — require("netlink.rt")() — inconsistent with socket.inet's inet.tcp(). This makes netlink a namespace like socket.inet:

local netlink = require("netlink")
local session = netlink.rt()          -- was require("netlink.rt")()
local g       = netlink.genl()
local ch      = netlink.channel(name) -- unchanged
local sock    = inet.tcp()            -- socket family: unchanged

require("netlink") now returns a table whose netlink.rt(), netlink.genl() and netlink.channel(name) are the factories, mirroring inet.tcp()/inet.udp(). The aggregator requires the three members eagerly; loading rt/genl only defines the classes (constructing a session is what needs a sleepable runtime), so a softirq runtime that only wants netlink.channel requires netlink all the same — the channel test exercises exactly that.

How

• The channel C module registers as netlink.channel (luaopen_netlink_channel); verified in-kernel that the dotted C module name resolves. The source stays lib/luanetlink.c and the kernel module stays luanetlink.ko.
• A small Lua aggregator (lib/netlink.lua) re-exports channel, rt, genl.

Channel callers (require("netlink").channel(name)) are unchanged. Full socket and netlink suites pass; LDoc builds clean.

Supersedes #622 (the .new approach): keeps everything callable and consistent with inet.tcp(), which is what the redesign was after.

Open branches (nl80211 #617, netfailover #618, floodguard #619) rebase onto this.

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