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

@@ -285,10 +285,11 @@ bool ProcStats::fetch( procInfo& f ) {
           &pinfo.kstkesp, &pinfo.kstkeip, &pinfo.signal, &pinfo.blocked, &pinfo.sigignore, &pinfo.sigcatch,
           &pinfo.wchan );
       // resident set size in pages
-      const auto pr_size   = static_cast<double>( pinfo.vsize );
-      const auto pr_rssize = static_cast<double>( pinfo.rss );
-      f.vsize              = pr_size / ( 1024 * 1024 );
-      f.rss                = pr_rssize * m_pg_size / ( 1024 * 1024 );
+      const auto       pr_size   = static_cast<double>( pinfo.vsize );
+      const auto       pr_rssize = static_cast<double>( pinfo.rss );
+      constexpr double MB        = 1.0 / ( 1024 * 1024 );
+      f.vsize                    = pr_size * MB;
+      f.rss                      = pr_rssize * m_pg_size * MB;
       if ( 0 == pinfo.vsize ) { ok = false; }
     }
     return ok;