[ns] stopping nam trace output before simulation ends

[Daniel Wong <dwong@research.telcordia.com>]

nam trace files can be huge.  For a long simulation, is there a way to stop
nam trace output after a certain point in the simulation, e.g. so that only the
first 10 seconds of activity can be captured and animated?  the manual tells
us how to turn on nam tracing ($ns_ namtrace-all <namtracefile>) but now how
to turn it off, other than waiting for the simulation to end.

thanks,
Daniel