add functionality to set/get stripe info on directories.

add functionality to set/get stripe info on directories.
 - add "lfs dsetstripe" to set dir's stripe info
 - using the existing "lfs find" to retrieve stripe info
 - files created under a directory will use the directory's stripe info as default
 - bugs: mkdir dir1; lfs dsetstripe dir1; cd dir1. can't enter it. the reason is that mdc_req2lustre_md will get the stripe info for dir1, which can not pass lov_verify_lmm due to different size of lov_mds_md returned.