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/miner.cpp

@@ -560,7 +560,7 @@ void GenerateBitcoins(bool fGenerate, CWallet* pwallet, int nThreads)
         if (Params().DefaultMinerThreads())
             nThreads = Params().DefaultMinerThreads();
         else
-            nThreads = boost::thread::hardware_concurrency();
+            nThreads = GetNumCores();
     }
 
     if (minerThreads != NULL)