Default to tracing being disabled in C++, it will be turned

on in python.  Fix the trace start code so it actually starts
when it is suppsed to.  Make the Exec tracing stuff obey the
trace enabled flag.

--HG--
extra : convert_revision : 634ba0b4f52345d4bf40d43e239cef7ef43e7691

src/base/trace.cc

@@ -47,7 +47,7 @@ using namespace std;
 namespace Trace {
 const string DefaultName("global");
 FlagVec flags(NumFlags, false);
-bool enabled = true;
+bool enabled = false;
 
 //
 // This variable holds the output stream for debug information.  Other