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

In non-stop mode, each thread is handled individually.  It should
be like you have 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 the stepping thread will have its state, and the
other thread will have its own state.

Every exec command should apply only to the selected thread.

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 a running thread.

Info threads now shows the running state of a thread.  MI
support can be added on top.

(gdb) info threads
  3 Thread 0xf7603b90 (LWP 23454)  (running)
* 2 Thread 0xf7e04b90 (LWP 23453)  0xffffe410 in __kernel_vsyscall ()
  1 Thread 0xf7e056b0 (LWP 23450)  (running)

-- 
Pedro Alves