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

@@ -261,10 +261,10 @@ ProcStats::ProcStats() {
 bool ProcStats::fetch( procInfo& f ) {
   if ( !m_valid ) { return false; }
 
-#if defined( __linux__ ) or defined( __APPLE__ )
-
   std::scoped_lock lock{ m_mutex };
 
+#if defined( __linux__ ) or defined( __APPLE__ )
+
   double     pr_size{ 0 }, pr_rssize{ 0 };
   linux_proc pinfo;
   int        cnt{ 0 };