Try to use secure_getenv() in preference to __secure_getenv()

Try to use secure_getenv() in preference to __secure_getenv()

If secure_getenv() use it in preference to __secure_getenv().
Starting with (e)glibc version 2.17, secure_getenv() exists, while
__secure_getenv() only works with shared library links (where it is a
weak symbol), but not for static links with /lib/libc.a

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>