[CPP_RPC] Replace legacy OS-specific API with std:: libraries

[cbalint13]

Replaces OS dependent API with standard cpp libraries in the tvm_rpc tool.

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• Replace all filesystem calls with safe and portable std::filesystem
• Worker process remains fork()-ed (POSIX), due to kill() enforcement on timeout
• Timer process is migrated from fork() to main with WNOHANG (nonblock) pid watcher
• Watch a single fork()-ed worker, no more "guess" on which fork() will "die/end" first
• Cleanup ambiguous doc entries, consistent default parameters across various places

Currently tested with real tuning processes but limited to linux machines.

[gemini-code-assist]

Code Review

This pull request modernizes the C++ RPC server by refactoring file system operations to use C++17 <filesystem> and replacing the double-fork watchdog mechanism with a cleaner thread-based watchdog. Feedback focuses on critical security and robustness improvements: restricting directory permissions from world-writable (0777) to owner-only (0700) to prevent local privilege escalation, resolving a potential race condition where a recycled PID could be killed by the watchdog thread, and using the std::error_code overload of std::filesystem::create_directories to avoid potential crashes from unhandled exceptions.

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[cbalint13]

This is ready to review.
Cc @tqchen