WRT binary link handling updated.

WRT binary link handling updated.

[Issue#] N/A
[Feature/Bug] N/A
[Problem] WRT application label should be retrieved from link to executable
xattrs. Currently widget_id is the label.
[Cause] N/A
[Solution] Smack label is retrieved from link to executable. app_label_dir
has been modified to label links to executable files with proper exec label.

[Verification] Run libprivilege-control-test. Reboot, install a widget and
launch it. http://slp-info.sec.samsung.net/gerrit/#/c/204855/

Change-Id: Iae87854283989f0d3ff0b76c9092c10654f47c7c