Fix PR18360 - internal error when using "interrupt -a"
    
    v2:
    
      - add handle_stop_requested and use it in all event waitkinds.
    
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
    If you do "interrupt -a" just while some thread is stepping over a
    breakpoint, gdb trips on an internal error.
    
    The test added by this patch manages to trigger this consistently by
    spawning a few threads that are constantly tripping on a conditional
    breakpoint whose condition always evaluates to false.  With current
    gdb, you get:
    
    ~~~
     interrupt -a
     .../src/gdb/inline-frame.c:343: internal-error: void skip_inline_frames(ptid_t): Assertion `find_inline_frame_state (ptid) == NULL' failed.
     A problem internal to GDB has been detected,
     further debugging may prove unreliable.
     Quit this debugging session? (y or n) FAIL: gdb.threads/interrupt-while-step-over.exp: displaced-stepping=on: iter=0: interrupt -a (GDB internal error)
    [...]
     .../src/gdb/inline-frame.c:343: internal-error: void skip_inline_frames(ptid_t): Assertion `find_inline_frame_state (ptid) == NULL' failed.
     A problem internal to GDB has been detected,
     further debugging may prove unreliable.
     Quit this debugging session? (y or n) FAIL: gdb.threads/interrupt-while-step-over.exp: displaced-stepping=off: iter=0: wait for stops (GDB internal error)
    ~~~
    
    The assertion triggers because we're processing a stop for a thread
    that had already stopped before and thus had already its inline-frame
    state filled in.
    
    Calling handle_inferior_event_1 directly within a
    "thread_stop_requested" observer is something that I've wanted to get
    rid of before, for being fragile.  Nowadays, infrun is aware of
    threads with pending events, so we can use that instead, and let the
    normal fetch_inferior_event -> handle_inferior_event code path handle
    the forced stop.
    
    The change to finish_step_over is necessary because sometimes a thread
    that was told to PTRACE_SINGLESTEP reports back a SIGSTOP instead of a
    SIGTRAP (i.e., we tell it to single-step, and then interrupt it quick
    enough that on the kernel side the thread dequeues the SIGTOP before
    ever having had a chance of executing the instruction to be stepped).
    SIGSTOP gets translated to a GDB_SIGNAL_0.  And then finish_step_over
    would miss calling clear_step_over_info, and thus miss restarting the
    other threads (which in this case of threads with pending events,
    means setting their "resumed" flag, so their pending events can be
    consumed).
    
    And now that we always restart threads in finish_step_over, we no
    longer need to do that in handle_signal_stop.
    
    Tested on x86_64 Fedora 23, native and gdbserver.
    
    gdb/ChangeLog:
    yyyy-mm-dd  Pedro Alves  <palves@redhat.com>
    
    	PR gdb/18360
    	* infrun.c (start_step_over, do_target_resume, resume)
    	(restart_threads): Assert we're not resuming a thread that is
    	meant to be stopped.
    	(infrun_thread_stop_requested_callback): Delete.
    	(infrun_thread_stop_requested): If the thread is internally
    	stopped, queue a pending stop event and clear the thread's
    	inline-frame state.
    	(handle_syscall_event, handle_inferior_event_1): Use
    	handle_stop_requested.
    	(handle_stop_requested): New function.
    	(handle_signal_stop): Set the thread's stop_signal here instead of
    	at caller.
    	(finish_step_over): Clear step over info unconditionally.
    	(handle_signal_stop): If the user had interrupted the event
    	thread, consider the stop a random signal.
    	(handle_signal_stop)
    	breakpoint>: Don't restart threads here.
    	(stop_waiting): Don't clear step-over info here.
    
    gdb/testsuite/ChangeLog:
    yyyy-mm-dd  Pedro Alves  <palves@redhat.com>
    
    	PR gdb/18360
    	* gdb.threads/interrupt-while-step-over.c: New file.
    	* gdb.threads/interrupt-while-step-over.exp: New file.