This patch adds the base inferior control support for non-stop mode.

In non-stop mode, each thread is handled individually.  It should
behave close to having a separate debugger attached to each thread.
To accomplish that, as soon as we have an event, we context switch
to it, and go on handling it.  The cases of hiting a breakpoint
in another thread while we're stepping don't need to be handled
specially, as each thread will have its own stepping state.

Another difference in non-stop mode, is that exec commands
apply only to the selected thread; the target is never told
to resume all threads -- much like with scheduler locking
on.

It is also needed to have support to interrupt/suspend a simple
thread so a new target_stop_ptid method was added to request
the target to interrupt a single thread.

Several checks have been added so GDB doesn't try to do
things with running threads, which don't make sense,
like asking for the current PC of an executing thread.

"info threads" now shows "(running)" instead of info on the
selected frame, on running threads.  MI support will be
added on top.

-- 
Pedro Alves