EX-5113 utils: add pcc pin/unpin commands

EX-5113 utils: add pcc pin/unpin commands

Store pin information in xattr trusted.pin.
Add lfs pcc pin/unpin commands to set/clear pcc pin flag.
A pcc-pinned file will not be removed from local storage
cache by lpcc_purge automatically.

Examples of xattr trusted.pin:
trusted.pin=[hsm: 2]
trusted.pin=[hsm: 2,hsm: 3]

Change-Id: I17ddf5ac8dc4eae48c0f6bdd0f2a19240474b0f5
Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial testlist=sanity-pcc
Reviewed-on: https://review.whamcloud.com/47083
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>