PR gdb/15713 - errors from i386_linux_resume lead to lock-up

PR gdb/15713 - errors from i386_linux_resume lead to lock-up

linux_nat_resume is not considering that linux_ops->to_resume may throw:

  /* Mark LWP as not stopped to prevent it from being continued by
     linux_nat_resume_callback.  */
  lp->stopped = 0;

  if (resume_many)
    iterate_over_lwps (ptid, linux_nat_resume_callback, NULL);

If something within linux_nat_resume_callback throws, GDB leaves the
lwp_info as if the inferior was resumed, while it actually wasn't.

A couple examples, there are possibly others:

 - i386_linux_resume calls target_read which calls QUIT.
 - if the actual ptrace resumption fails in inf_ptrace_resume,
   perror_with_name is called.

If the user tries to kill the inferior at this point (or quit, which
offers to kill), GDB locks up trying to stop the lwp -- if it is
already stopped no new waitpid event gets generated for it.

Fix this by setting the stopped flag earlier, as soon as we collect a
stop event with waitpid, and clearing it always only after resuming
the lwp successfully.

Tested on x86_64 Fedora 20.  Confirmed the lock-up disappears using a
local hack that forces an error in inf_ptrace_resume.

Also fixes a little "set debug lin-lwp" annoyance.  Currently we always see:

 Continuing.
 LLR: Preparing to resume process 6802, 0, inferior_ptid Thread 0x7ffff7fc7740 (LWP 6802)
                                                                                ^^^^^^^^
 RC: Resuming sibling Thread 0x7ffff77c5700 (LWP 6807), 0, resume
 RC: Resuming sibling Thread 0x7ffff7fc6700 (LWP 6806), 0, resume
 RC: Not resuming sibling Thread 0x7ffff7fc7740 (LWP 6802) (not stopped)
                                                 ^^^^^^^^^^^^^^^^^^^^^^^
 LLR: PTRACE_CONT process 6802, 0 (resume event thread)

This patch gets rid of the "Not resuming sibling" line.

2014-05-29  Pedro Alves  <palves@redhat.com>

	PR gdb/15713
	* linux-nat.c (linux_nat_resume_callback): Rename the second
	parameter to 'except'.  Skip LP if it points to EXCEPT.
	(linux_nat_resume): Don't mark the event lwp as not stopped
	before resuming sibling lwps.  Instead ask
	linux_nat_resume_callback to skip the event lwp.  Mark it as not
	stopped after actually resuming it.
	(linux_handle_syscall_trap): Mark the lwp as not stopped after
	resuming it.
	(wait_lwp): Mark the lwp as stopped here.
	(stop_wait_callback): Mark the lwp as not stopped right after
	resuming it.  Don't mark lwps as stopped here.
	(linux_nat_filter_event): Mark the lwp as stopped earlier.
	(linux_nat_wait_1): Don't mark dead lwps as stopped here.