Implement AT_SECURE properly.

AT_SECURE auxv entry has been added to the Linux 2.5 kernel to pass a
boolean flag indicating whether secure mode should be enabled. 1 means
that the program has changes its credentials during the execution.
Being exported AT_SECURE used by glibc issetugid() call.

Submitted by: imp, dchagin
Security: FreeBSD-SA-16:10.linux
Security: CVE-2016-1883