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>