LU-13023 pcc: Incorrect size after re-attach

LU-13023 pcc: Incorrect size after re-attach

The following test case will result in incorrect size for PCC copy:
- Attach a file with size of s1 (s2 > 0) into PCC;
- Detach this file with --keep option, and the data will retain
  on PCC;
- Truncate this file locally or on an remote client to a new size
  s2 (s2 < s1);
- Re-attach the file again. The size of PCC copy is still s1.

To solve this problem, it need to truncate the size of the PCC copy
to the same size of the Lustre copy which will be HSM released
later after finished the data copy (archive) phase.
This patch also adds the handle for the signal pending when the
attach process is killed by an administrator.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I18f2c883454450bf5dc2f2b3600e2685d8f8f130
Reviewed-on: https://review.whamcloud.com/36884
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>