perf improvement in get_process_info for Linux

[ZhidongPeng]

The get_process_info function creates a brand new System::new_with_specifics for every single process lookup. This is extremely inefficient because:

• System::new_with_specifics with RefreshKind::new().with_processes(...) scans the entire proc filesystem to enumerate ALL processes
• This happens for every network connection that needs to be authorized
• The perf profile shows this is spending ~80% of CPU time on syscalls related to proc access

The fix is to replace the sysinfo::System approach with direct /proc/{pid}/ reads, which is much more efficient since we only need to read information for a specific PID, not scan all processes.

Improvement:
Before fix: Captured 3493 samples (0.590 MB)
After fix: Captured 121 samples (0.049 MB)