Allow top(1) to toggle display of processes+threads or *only* threads

Allow top(1) to toggle display of processes+threads or *only* threads
by adding a '-O' command line argument and run-time toggle.

This is useful when one wants to only monitor activity of current
active threads.

Document this new functionality and update the patches accordingly.

Requested-by:	David Rhodus <drhodus@machdep.com>