Fix for Coverity perl5 CID 29068: Insecure temporary file (SECURE_TEMP) secure_temp...

Fix for Coverity perl5 CID 29068: Insecure temporary file (SECURE_TEMP) secure_temp: Calling mkstemp() without securely setting umask first.

The umask used for mkstemp should be secure, but umask 0600 has been
the required umask only since POSIX.1-2008.  In glibc 2.06 and earlier
the default was 0666, which is not secure.  And no explicit knowledge
of how well non-glibc platforms implement mkstemp.  Better err on the
side security, so set the umask temporarily to 0600, and then restore it.