Set signal to 0 after enqueue_pending_signal

Set signal to 0 after enqueue_pending_signal

Today, we enqueue signal in linux_resume_one_lwp_throw, but set
variable 'signal' many lines below with the comment

      /* Postpone any pending signal.  It was enqueued above.  */
      signal = 0;

I feel difficult to associate code across many lines, and we should
move the code close to enqueue_pending_signal call.  This is what
this patch does in general.  After this change, variable 'signal'
is set to zero very early, so the 'signal' value in the following
debugging message makes no sense, so I remove it from the debugging
message.  The function returns early if lwp->status_pending_p is
true, so 'signal' value in the debugging message doesn't matter,
AFAICS.  Also, I move one debugging message several lines below to
make it close the real ptrace call,

  if (debug_threads)
    debug_printf ("Resuming lwp %ld (%s, signal %d, stop %s)\n",
		  lwpid_of (thread), step ? "step" : "continue", signal,
		  lwp->stop_expected ? "expected" : "not expected");

so that the debugging message can reflect what GDBserver does.  This
is a code refactor and only debugging messages are affected.

gdb/gdbserver:

2016-03-18  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (linux_resume_one_lwp_throw): Set 'signal' to
	0 if signal is enqueued.  Remove 'signal' from one debugging
	message.  Move one debugging message to some lines below.
	Remove code setting 'signal' to 0.