Provide a function for launchers for dropping process capabilities

Provide a function for launchers for dropping process capabilities

The functions for launchers, manipulating process Smack label and groups,
require elevated privileges. Since they will be called by launcher after
fork, in the process for the application, privileges should be dropped
before running an actual application.
This patch introduces a convenience function for launchers for dropping
capabilities from a process: security_manager_drop_process_privileges.

Change-Id: Iff06554bdcf2d51d0163e4dcb83ea9b976896740
Signed-off-by: Rafal Krypa <r.krypa@samsung.com>