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