The current comment implies that we only strip sensitive
environment variables on executing a setuid program. This is
true but incomplete. The AT_SECURE flag is set whenever a
security transition occurs, such as executing a setuid program,
SELinux security transition, executing a file with file capabilities,
etc...
Fixup the comments.
Change-Id: I30a73992adfde14d6e5f642b3a1ead2ee56726be
2fb02651c8