Add flag MD_DISK_REPLACEMNT in kernel

Hi Neil,
	From your latest mdadm-git-tree, i found you add func to support relacement. Thanks your work!
	Review your code, you judge the spare is replacing through the field of superblock.
	I think, it also can through ioctl(GET_DISK_INFO,).
	If so, the detail command can do clearly(using replaceing instead of recovering).
	How about this? Or you have done it?

Thanks!
Jianpeng