LU-11188 lfs: add "--perm" option to "lfs find"

LU-11188 lfs: add "--perm" option to "lfs find"

Add support for "--perm" option to "lfs find".
The option supports both octal and symbolic representation and
follows the POSIX standard.
As for GNU find, it supports '-' and '/' modifiers before the
permission.

Signed-off-by: Guillaume Courrier <guillaume.courrier@cea.fr>
Change-Id: I8e1292421986c3a4bde686f3c7dc7bfcb679cabc
Reviewed-on: https://review.whamcloud.com/43715
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>