The "running" thread property recently added represents the frontend view
of the thread running state.  While doing a step for instance, the inferior 
starts/stops several times while single-stepping.  We only clear
the "running" state of the inferior when it does a normal_stop.
This patch adds a new field, that altough similar to "running" 
has a different usage.  It replaces the current target_executing
global with a similar per-thread concept.  Inferior and frame/stack
control then can then use this property to know when it is possible
to read registers and memory from a thread.

-- 
Pedro Alves