ProcStats : Use mutex lock to make fetch() thread safe.

Whilst investigating a possible memory issue using the Memory Auditor, I discovered the information printed was garbled if multiple threads where used.

The issue was the ProcStats::fetch method which was not thread safe. This MR fixes that with a simple mutex lock.

I also did a drive-by cleanup and code modernisation.

FYI @clemenci

GaudiAud/src/ProcStats.cpp

@@ -267,11 +267,11 @@ bool ProcStats::fetch( procInfo& f ) {
 
 #if defined( __linux__ ) or defined( __APPLE__ )
 
-  std::scoped_lock lock{m_mutex};
+  std::scoped_lock lock{ m_mutex };
 
-  double     pr_size{0}, pr_rssize{0};
+  double     pr_size{ 0 }, pr_rssize{ 0 };
   linux_proc pinfo;
-  int        cnt{0};
+  int        cnt{ 0 };
 
   m_ufd.lseek( 0, SEEK_SET );