Call shmget() with permission 0600 instead of 0777

A security advisory (TALOS-2019-0857/CVE-2019-5068) found that
creating shared memory regions with permission mode 0777 could allow
any user to access that memory.  Several Mesa drivers use shared-
memory XImages to implement back buffers for improved performance.

This path changes the shmget() calls to use 0600 (user r/w).

Tested with legacy Xlib driver and llvmpipe.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
(cherry picked from commit 02c3dad0)