Apply const qualifier to some users of bp_location.

	Apply const qualifier to some users of bp_location.
	* breakpoint.h (struct bpstats): Make
	the breakpoint_at field point at const bp_location.
	* breakpoint.c (bpstat_alloc): Accept const
	bp_location.
	(breakpoint_here_p, breakpoint_inserted_here_p)
	(software_breakpoint_inserted_here_p)
	(breakpoint_thread_match, bpstat_stop_status)
	(read_memory_nobpt, bpstat_have_active_hw_watchpoints): Use
	const bp_location for iteration.
	(print_it_typical, print_bp_stop_message): Use
	const bp_location variable.