* sysdeps/generic/enbl-secure.c (__libc_enable_secure_decided): New
variable.
(__libc_init_secure): Don't do anything if __libc_enable_secure_decided
is nonzero.
* include/unistd.h: Declare __libc_enable_secure_decided.
* elf/dl-support.c (_dl_aux_init): Recognize AT_UID, AT_EUID, AT_GID,
and AT_EGID. If all found, set __libc_enable_secure and
__libc_enable_secure_decided.