ifcfg,keyfile: fix temporary file races (CVE-2016-0764)

ifcfg,keyfile: fix temporary file races (CVE-2016-0764)

Two of these raised Coverity's eyebrows.

CID 59389 (#1 of 1): Insecure temporary file (SECURE_TEMP)
5.  secure_temp: Calling mkstemp without securely setting umask first.

CID 59388 (#1 of 1): Insecure temporary file (SECURE_TEMP)
1.  secure_temp: Calling mkstemp without securely setting umask first.

Last one raised mine.

When a connection is edited and saved, there's a small window during which and
unprivileged authenticated local user can read out connection secrets (e.g. a
VPN or Wi-Fi password). The security impact is perhaps of low severity as
there's no way to force another user to save their connection.