gallium/winsys: replace calls to dup(2) with fcntl(F_DUPFD_CLOEXEC)

gallium/winsys: replace calls to dup(2) with fcntl(F_DUPFD_CLOEXEC)

Without this fix, duplicated file descriptors leak into child processes.
See commit aaac913e901229d11a1894f6aaf646de6b1a542c for one instance
where the same fix was employed.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Matt Whitlock <freedesktop@mattwhitlock.name>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>