PR threads/18600: Inferiors left around after fork+thread spawn

PR threads/18600: Inferiors left around after fork+thread spawn

The new gdb.threads/fork-plus-threads.exp test exposes one more
problem.  When one types "info inferiors" after running the program,
one see's a couple inferior left still, while there should only be
inferior #1 left.  E.g.:

 (gdb) info inferiors
   Num  Description       Executable
   4    process 8393      /home/pedro/bugs/src/test
   2    process 8388      /home/pedro/bugs/src/test
 * 1    <null>            /home/pedro/bugs/src/test
 (gdb) info threads

Calling prune_inferiors() manually at this point (from a top gdb) does
not remove them, because they still have inf->pid != 0 (while they
shouldn't).  This suggests that we never mourned those inferiors.

Enabling logs (master + previous patch) we see:

 ...
 WL: waitpid Thread 0x7ffff7fc2740 (LWP 9513) received Trace/breakpoint trap (stopped)
 WL: Handling extended status 0x03057f
 LHEW: Got clone event from LWP 9513, new child is LWP 9579
 [New Thread 0x7ffff37b8700 (LWP 9579)]
 WL: waitpid Thread 0x7ffff7fc2740 (LWP 9508) received 0 (exited)
 WL: Thread 0x7ffff7fc2740 (LWP 9508) exited.
			    ^^^^^^^^
 [Thread 0x7ffff7fc2740 (LWP 9508) exited]
 WL: waitpid Thread 0x7ffff7fc2740 (LWP 9499) received 0 (exited)
 WL: Thread 0x7ffff7fc2740 (LWP 9499) exited.
 [Thread 0x7ffff7fc2740 (LWP 9499) exited]
 RSRL: resuming stopped-resumed LWP Thread 0x7ffff37b8700 (LWP 9579) at 0x3615ef4ce1: step=0
 ...
 (gdb) info inferiors
   Num  Description       Executable
   5    process 9508      /home/pedro/bugs/src/test
		^^^^
   4    process 9503      /home/pedro/bugs/src/test
   3    process 9500      /home/pedro/bugs/src/test
   2    process 9499      /home/pedro/bugs/src/test
 * 1    <null>            /home/pedro/bugs/src/test
 (gdb)
 ...

Note the "Thread 0x7ffff7fc2740 (LWP 9508) exited." line.
That's this in wait_lwp:

      /* Check if the thread has exited.  */
      if (WIFEXITED (status) || WIFSIGNALED (status))
	{
	  thread_dead = 1;
	  if (debug_linux_nat)
	    fprintf_unfiltered (gdb_stdlog, "WL: %s exited.\n",
				target_pid_to_str (lp->ptid));
	}
    }

That was the leader thread reporting an exit, meaning the whole
process is gone.  So the problem is that this code doesn't understand
that an WIFEXITED status of the leader LWP should be reported to
infrun as process exit.

gdb/ChangeLog:
2015-07-30  Pedro Alves  <palves@redhat.com>

	PR threads/18600
	* linux-nat.c (wait_lwp): Report to the core when thread group
	leader exits.

gdb/testsuite/ChangeLog:
2015-07-30  Pedro Alves  <palves@redhat.com>

	PR threads/18600
	* gdb.threads/fork-plus-threads.exp: Test that "info inferiors"
	only shows inferior 1.