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

@@ -256,8 +256,6 @@ void ProcStats::open_ufd() {
 #endif // __linux__ or __APPLE__
 }
 
-ProcStats::ProcStats() { open_ufd(); }
-
 bool ProcStats::fetch( procInfo& f ) {
   if ( !m_valid ) { return false; }