gdbserver: fix killed-outside.exp

killed-outside.exp regresses with "maint set target-non-stop on".  The
logs show:

 (gdb) continue
 Continuing.
 infrun: clear_proceed_status_thread (Thread 9028.9028)
 infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
 infrun: proceed: resuming Thread 9028.9028
 Sending packet: $Z0,3615a03966,1#4b...  Notification received: Stop:X9;process:2344
 Packet received: E01
 Sending packet: $Z0,3615a13970,1#47...Packet received: E01
 Sending packet: $Z0,3615a14891,1#4a...Packet received: E01
 infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [Thread 9028.9028] at 0x4005e4
 Sending packet: $vCont;c:p2344.2344#1a...Packet received: E.target not running.
 Sending packet: $qXfer:threads:read::0,fff#03...Packet received: l<threads>\n</threads>\n
 Sending packet: $vStopped#55...Packet received: OK
 Unexpected vCont reply in non-stop mode: E.target not running.
 (gdb) remote_async_inferior_event_handler
 infrun: target_wait (-1.0.0, status) =
 infrun:   9028.0.0 [process 9028],
 infrun:   status->kind = signalled, signal = GDB_SIGNAL_KILL
 infrun: TARGET_WAITKIND_SIGNALLED

 Program terminated with signal SIGKILL, Killed.
 The program no longer exists.
 infrun: stop_waiting
 infrun: clear_step_over_info
 infrun: stop_all_threads
 remote_thread_exit_events(1)

Note the "Unexpected vCont reply" error.

I traced it to a problem in status_pending_p_callback.  It resumes an
LWP when it shouldn't.

gdb/gdbserver/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* linux-low.c (thread_still_has_status_pending_p): Don't check
	vCont;t here.
	(lwp_resumed): New function.
	(status_pending_p_callback): Return early if the LWP is not
	supposed to be resumed.