metrics: per-client read timeout and max-clients cap

A scrape client that opens a TCP connection and never sends a full
request line+headers used to keep its accel-pppd-side fd registered
indefinitely. Combined with the default `allowed_ips` (= allow all),
a single peer could exhaust the daemon's file descriptors
slowloris-style.

Give every accepted connection a triton timer armed for
`read_timeout` seconds (default 5). On expiry, disconnect_client()
tears down the fd, the timer, and the buffer. The timer is canceled
implicitly when the client is disconnected for any other reason
because disconnect_client() now deletes the timer before freeing the
client.

Also cap the number of in-flight clients at `max_clients`
(default 64). Excess connections are accepted and immediately
closed so the kernel listen backlog still drains.

Both knobs accept 0 to disable. The default values are documented in
accel-ppp.conf(5) alongside the existing [metrics] options.

Smoke-tested:

  * a connection that sends nothing is dropped from the daemon's fd
    table when read_timeout elapses; subsequent scrapes still
    succeed;
  * with max_clients=3 and five concurrent silent connections, the
    daemon holds exactly three ESTAB sockets, the others are closed.

Signed-off-by: Denys Fedoryshchenko <denys.f@collabora.com>

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