add a flag timezero to trnoise_state

falg is true as long as time = 0. Allows
to detect the change from time == 0 to time > 0
only then the 1/f data may be created anew (e.g.
due to multiple 'run' commands

src/include/ngspice/1-f-code.h

@@ -17,6 +17,8 @@ struct trnoise_state
     double *oneof;
     size_t oneof_length;
 
+    bool timezero;
+
     double RTScapTime, RTSemTime;
     bool RTS;
 };