* inf-ttrace.c (inf_ttrace_follow_fork): Register the main thread

	* inf-ttrace.c (inf_ttrace_follow_fork): Register the main thread
	of the child fork.
	(inf_ttrace_attach): Add the main thread.
	(inf_ttrace_resume_callback): Check for exited threads.  Adjust
	for always a thread.
	(inf_ttrace_wait): Decorate the main thread's ptid with lwp info
	using thread_change_ptid, and set its private data.  Don't add the
	main thread here.
	(inf_ttrace_pid_to_str): Adjust.