LU-14398 lfs: use llapi_fid2path_at() in lfs_fid2path()

LU-14398 lfs: use llapi_fid2path_at() in lfs_fid2path()

Use llapi_fid2path_at() in lfs_fid2path(). This avoids resolving and
opening the mount point for each FID argument passed. Make the -c,
--cur, --current option actually print the link. Add a more
descriptive long option name for this (--print-link). Update the
lfs-fid2path manpgae accordingly.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: If851e4ce95f87d3188b644eb4a345ba3cfca530d
Reviewed-on: https://review.whamcloud.com/41407
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>