Fix CVE-2011-4029: File permission change vulnerability.

Use fchmod() to change permissions of the lock file instead
of chmod(), thus avoid the race that can be exploited to set
a symbolic link to any file or directory in the system.

Signed-off-by: Matthieu Herrb <matthieu.herrb@laas.fr>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
(cherry picked from commit b67581cf)
(cherry picked from commit 12f65819)