More tolerate port group support, plugin cleanup.

More tolerate port group support, plugin cleanup.

Gracefully handle plugins with illegal groups that contain no symbol by
treating ports in that group normally (i.e. as if they weren't part of a
group).  The port groups spec mandates that groups have a valid unique
symbol, but plugins aren't perfect...

Make (conceptually) static plugin variables actually (C) static,
and clean up SLV2 related things with a GCC shared library destructor,
if GCC is in use.