Commands: ensure the CommandDef is not messed up by copying

...better make it noncopyable to enforce the builder-style use.

In the recent test, I observed strange behaviour when erroneously passing
the CommandDef by value; the command seemed to be registered just fine,
but afterwards, the registry was empty. I must admit I don't understand
this, just from reading the code in CommandDef and Command it should
work just fine to activate a copy of the originally started CommandDef;
anyway, I didn't care to track that issue down, rather make the
CommandDef noncopyable as it should have been right from start.

src/proc/control/command-def.hpp

@@ -71,6 +71,7 @@
 
 #include <memory>
 #include <functional>
+#include <boost/noncopyable.hpp>
 
 
 
@@ -276,7 +277,8 @@ namespace control {
    * 
    */
   class CommandDef
-    : public lib::BoolCheckable<CommandDef>
+    : public lib::BoolCheckable<CommandDef
+    , boost::noncopyable       >
     {
       Symbol id_;
       Command prototype_;