Use real number of cores for default -par, ignore virtual cores

To determine the default for `-par`, the number of script verification
threads, use [boost::thread::physical_concurrency()](http://www.boost.org/doc/libs/1_58_0/doc/html/thread/thread_management.html#thread.thread_management.thread.physical_concurrency)
which counts only physical cores, not virtual cores.

Virtual cores are roughly a set of cached registers to avoid context
switches while threading, they cannot actually perform work, so spawning
a verification thread for them could even reduce efficiency and will put
undue load on the system.

Should fix issue #6358, as well as some other reported system overload
issues, especially on Intel processors.

The function was only introduced in boost 1.56, so provide a utility
function `GetNumCores` to fall back for older Boost versions.

src/qt/optionsdialog.cpp

@@ -42,7 +42,7 @@ OptionsDialog::OptionsDialog(QWidget *parent, bool enableWallet) :
     /* Main elements init */
     ui->databaseCache->setMinimum(nMinDbCache);
     ui->databaseCache->setMaximum(nMaxDbCache);
-    ui->threadsScriptVerif->setMinimum(-(int)boost::thread::hardware_concurrency());
+    ui->threadsScriptVerif->setMinimum(-GetNumCores());
     ui->threadsScriptVerif->setMaximum(MAX_SCRIPTCHECK_THREADS);
 
     /* Network elements init */