don't call add_target for thread_db_ops

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.