Use secure_getenv(3) when available.

Change all calls to getenv() inside libcrypto to use a new wrapper function
that use secure_getenv() if available and an issetugid then getenv if not.

CPU processor override flags are unchanged.

Extra checks for OPENSSL_issetugid() have been removed in favour of the
safe getenv.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7047)