lib/env, ...: use getauxval(AT_SECURE) for SUID check

lib/env, ...: use getauxval(AT_SECURE) for SUID check

Comparing effective and real uid/gid is not a proper way to check for
SUID execution:

1. this does not consider file capabilities

2. this check breaks when NO_NEW_PRIVS is used as the Linux kernel
   resets effective ids during execve(); this means the check is
   false, but the process still has raised capabilities

For more details about the NO_NEW_PRIVS problem, check this post and
the surrounding thread:

 https://lore.kernel.org/lkml/20250509184105.840928-1-max.kellermann@ionos.com/

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>