* target.h (struct target_ops) <to_supports_multi_process>: New

	* target.h (struct target_ops) <to_supports_multi_process>: New
	field.
	(target_supports_multi_process): New define.
	* target.c (update_current_target): Inherit and de_fault
	to_supports_multi_process.
	* infcmd.c (attach_command): Allow attaching to multiple processes
	if the target supports it.
	(detach_command): If the target claims there is still execution,
	don't clear the thread list.
	* remote.c (remote_supports_multi_process): New.
	(init_remote_ops): Register remote_supports_multi_process.