vm: only show factorbug help the first time

vm/debug.cpp

@@ -396,7 +396,10 @@ void factor_vm::factorbug()
 	fep_p = true;
 
 	std::cout << "Starting low level debugger..." << std::endl;
-	factorbug_usage(false);
+	if (!fep_help_was_shown) {
+		factorbug_usage(false);
+		fep_help_was_shown = true;
+	}
 	bool seen_command = false;
 
 	for(;;)

vm/vm.cpp

@@ -18,6 +18,7 @@ factor_vm::factor_vm() :
 	current_gc(NULL),
 	gc_events(NULL),
 	fep_p(false),
+	fep_help_was_shown(false),
 	fep_disabled(false),
 	full_output(false),
 	last_nano_count(0),

vm/vm.hpp

@@ -111,6 +111,7 @@ struct factor_vm
 
 	/* Debugger */
 	bool fep_p;
+	bool fep_help_was_shown;
 	bool fep_disabled;
 	bool full_output;