Simplify temp movie directory creation

plot_io_movie() was calling create_movie_temp_dir() which unnecessarily
strdup()ed a string constant leaving plot_io_movie() to free it. Replace
the strdup() with a mutable char array and get rid of the free(). Merge
the few remaining lines which create the movie dir into plot_io_movie().

Also prune a duplicate declaration of start_mpstat() in tracers.h

Signed-off-by: Andrew Price <anprice@redhat.com>