securebits: some cleanup

* Remove .sh and divide into three seperate tests.

* Check the header <linux/securebits.h>.

* Use tst_brkm instead of tst_exit() that follows tst_resm.

* Some cleanup

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>