fix(dlt-receive): set SO_RCVTIMEO and SO_KEEPALIVE to enable -r reconnect on half-open TCP

[aki1770-del]

fix(dlt-receive): set SO_RCVTIMEO and SO_KEEPALIVE in dlt_client_connect to enable -r reconnect on half-open TCP

Fixes #817.

Problem

dlt-receive -r <interval> is supposed to reconnect automatically when the
DLT daemon becomes unavailable. This works correctly for clean disconnects
(RST/FIN received): dlt_receiver_receive() returns ≤0, dlt_client_main_loop
exits, and the reconnect loop fires after the specified interval.

However, when the network path fails silently — half-open TCP, common in
VM/container restarts and network partitions — the kernel socket stays in
ESTABLISHED state. dlt_receiver_receive() calls recv() which blocks
forever. The reconnect interval is never consulted.

Fix

Three files changed:

include/dlt/dlt_client.h: Add recv_timeout_sec field to DltClient
struct (0 = no timeout, backwards-compatible default).

src/lib/dlt_client.c — in dlt_client_connect(), after blocking mode
is restored on a successful TCP connect:

• If client->recv_timeout_sec > 0: call setsockopt(SO_RCVTIMEO) with the
  configured timeout. When recv() times out it returns -1/EAGAIN, which
  dlt_receiver_receive() maps to a ≤0 return, causing dlt_client_main_loop
  to exit and the reconnect loop to fire.
• Always set SO_KEEPALIVE (+ TCP_KEEPIDLE/TCP_KEEPINTVL/TCP_KEEPCNT
  where available) as a second line of defence.

src/console/dlt-receive.c — before the reconnect loop: when -r is
active, set dltclient.recv_timeout_sec = rvalue / 1000 (minimum 5 s) so
the socket timeout matches the reconnect interval.

Behaviour

Scenario	Before	After
Clean disconnect (RST/FIN)	Reconnect works	Unchanged
Half-open TCP (-r not set)	recv() blocks forever	recv() blocks forever (no change — timeout = 0)
Half-open TCP (-r 5000 = 5 s)	recv() blocks forever, -r ignored	recv() times out after 5 s, reconnect fires

AI-assisted — authored with Claude, reviewed by Komada.