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/util.h

@@ -199,6 +199,13 @@ std::string HelpMessageGroup(const std::string& message);
  */
 std::string HelpMessageOpt(const std::string& option, const std::string& message);
 
+/**
+ * Return the number of physical cores available on the current system.
+ * @note This does not count virtual cores, such as those provided by HyperThreading
+ * when boost is newer than 1.56.
+ */
+int GetNumCores();
+
 void SetThreadPriority(int nPriority);
 void RenameThread(const char* name);