LUDOC-514: add --acls and -P for MDT backup/restore

LUDOC-514: add --acls and -P for MDT backup/restore

Tar will only restore the following xattr if --acls is provided:
system.posix_acl_access
system.posix_acl_default

Update the documentation to mention --acls for MDT backup/restore.

Lastly, using -P / --absolute-names speeds up tremendously the
extraction for trusted archive with hardlinks (like MDT backup
archives).

Signed-off-by: Stephane Thiell <sthiell@stanford.edu>
Change-Id: I9db6729c029580edaf77129fc752befb6f0dc9fb
Reviewed-on: https://review.whamcloud.com/c/doc/manual/+/51599
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>