Add "kira track" command for particle tracking.

Add ring markers for non-isolated stars.  Not finished yet, but seems usable.
No GUI yet, but new commands:
   kiractl nodes  {on|off|roots}
   kiractl rings  {on|off|roots}
   kiractl ringsize {sep|semi}   (ring size from curr. separation or semimajor axis)
   kiractl scale  <scalefactor>
   kiractl span   <minpixels> <maxpixels>   (allowed range of ring radii)

Change encoding of nclump: n for leaves, -n for center-of-mass nodes,
   so colormap can make them distinguishable.

Bug: after kiractl command, must change timestep before it takes effect.
Bug: ring markers aren't colored yet for some reason.

Reduce SPECKCHUNK to 2001, not 10001.  Too many machines have tiny stacks.

User-adjustable wraparound time (but no way to adjust it yet).
Recompute colors and sizes whenever we update dynamic data.
Don't update dynamic data if time hasn't changed.
Discard old specks when updating dynamic data -- but allow for
keeping "ghost" copies (up to st->nghosts) for making trails.
We don't do trails yet though.

Lots of changes to support new time flow and additional
time-related GUI.  Probably not all there but at least somewhat usable.

Don't report error if WORDS_BIGENDIAN is undefined -- config.h might
just /* #undef */ it.  Just assume undefined -> false.

We're little-endian if WORDS_BIGENDIAN is defined (in config.h) and set to 0,
so use #if rather than #ifdef to test endian-ness.