Improve config file sync during configuration save.

Add fsync() calls to force sync to disk while saving configuration
file.  It is necessary to do a sync on the temp file before rename,
and to sync again on its parent directory to ensure both the
file content and the directory are up-to-date.

Change-Id: I3a862ad59c8ae5beb8ea2c727eb5f275a2d5823e