[Draft] Linux idle helper RPC probe + cc_config knobs (fixes #1187 path)

[nicepopo86-lang]

Summary

Phase-1 client-side integration for Linux idle helper RPC (RFC spike for #1187):

• adds optional UNIX-socket probe in HOST_INFO::user_idle_time() Linux path
  • request: GET_IDLE_SECS\n
  • response parse: OK <seconds> <source>
  • short socket timeout and safe fallback to existing logic when unavailable
• keeps existing XScreenSaver (xss_idle()) fallback path unchanged
• adds minimal cc_config.xml knobs:
  • <enable_idle_helper_rpc> (default 0)
  • <idle_helper_socket> (default /run/user/%d/boinc/idle-helper.sock)
  • <idle_helper_timeout_ms> (default 50)
• wires parsing/writing/showing of new config fields in both config parsers

Notes

• Defaults preserve current behavior unless helper RPC is explicitly enabled.
• Could not run a full compile in this environment because bootstrap dependency m4 is missing (./_autosetup fails before build).

Refs: #1187

---

Summary by cubic

Adds an optional Linux idle helper RPC probe over a UNIX socket with new cc_config knobs, keeping current idle detection as the default. Supports #1187 by querying a helper and safely falling back when unavailable.

• New Features

  • Linux: probe idle via UNIX socket (default /run/user/%d/boinc/idle-helper.sock) with short timeout and safe fallback.
  • New cc_config options: enable_idle_helper_rpc (false), idle_helper_socket, idle_helper_timeout_ms (50). Parsed, written, and shown in logs.
  • Idle time now uses min(existing, helper result). XScreenSaver path unchanged; debug logs behind idle_detection_debug.

• Migration

  • Off by default. To enable, set <enable_idle_helper_rpc>true</enable_idle_helper_rpc> in cc_config.xml; optionally set <idle_helper_socket> and <idle_helper_timeout_ms>.

^{Written for commit d3d5319. Summary will update on new commits.}

[AenBleidd]

See #6242