add a --max-concurrency flag

It sets the max number of threads to use for a parallel job.
This is different that the number of total threads, since monero
binaries typically start a lot of them.

src/common/util.cpp

@@ -422,4 +422,27 @@ std::string get_nix_version_display_string()
     umask(mode);
 #endif
   }
+
+  namespace
+  {
+    boost::mutex max_concurrency_lock;
+    unsigned max_concurrency = boost::thread::hardware_concurrency();
+  }
+
+  void set_max_concurrency(unsigned n)
+  {
+    if (n < 1)
+      n = boost::thread::hardware_concurrency();
+    unsigned hwc = boost::thread::hardware_concurrency();
+    if (n > hwc)
+      n = hwc;
+    boost::lock_guard<boost::mutex> lock(max_concurrency_lock);
+    max_concurrency = n;
+  }
+
+  unsigned get_max_concurrency()
+  {
+    boost::lock_guard<boost::mutex> lock(max_concurrency_lock);
+    return max_concurrency;
+  }
 }

src/common/util.h

@@ -160,4 +160,7 @@ namespace tools
   };
 
   void set_strict_default_file_permissions(bool strict);
+
+  void set_max_concurrency(unsigned n);
+  unsigned get_max_concurrency();
 }