Add simple profiling class to accumulate Stmt usage stats across runs.

Use the BROFILER_FILE environment variable to point to a file in
which Stmt usage statistics from Bro script-layer can be output.

This should be able to be used to check Bro script coverage that
that e.g. the entire test suite covers.

src/Stmt.cc

@@ -258,6 +258,7 @@ static BroFile* print_stdout = 0;
 
 Val* PrintStmt::DoExec(val_list* vals, stmt_flow_type& /* flow */) const
 	{
+	RegisterAccess();
 	if ( ! print_stdout )
 		print_stdout = new BroFile(stdout);