don't call add_target for thread_db_ops

Right now, "help target" will include this line:

    target multi-thread -- Threads and pthreads support

However, it doesn't make sense to invoke "target multi-thread".

This patch fixes the problem by not registering the multi-thread
target.  add_target does some needed initialization of the target_ops,
so I broke this out into a new function.

It isn't clear to me whether this patch requires a test case or not.
I'm not sure whether there are other unregistered targets; but if
there are, it seems unlikely that we test for their absence from the
help.

Built and regtested on x86-64 Fedora 18.

	* linux-thread-db.c (init_thread_db_ops): Call
	complete_target_initialization.
	(_initialize_thread_db): Don't call add_target.
	* target.c (complete_target_initialization): New function.
	(add_target_with_completer): Call it.
	* target.h (complete_target_initialization): Declare.

gdb/linux-thread-db.c

@@ -2057,6 +2057,8 @@ init_thread_db_ops (void)
   thread_db_ops.to_extra_thread_info = thread_db_extra_thread_info;
   thread_db_ops.to_get_ada_task_ptid = thread_db_get_ada_task_ptid;
   thread_db_ops.to_magic = OPS_MAGIC;
+
+  complete_target_initialization (&thread_db_ops);
 }
 
 /* Provide a prototype to silence -Wmissing-prototypes.  */
@@ -2066,7 +2068,6 @@ void
 _initialize_thread_db (void)
 {
   init_thread_db_ops ();
-  add_target (&thread_db_ops);
 
   /* Defer loading of libthread_db.so until inferior is running.
      This allows gdb to load correct libthread_db for a given