weston: Set CLOEXEC on stdin

We don't want to leak this into apps launched from the panel.

stdout and stderr are left for now because some things launched
by weston - such as weston-keyboard - share weston's log by
printing to those fds.

I'm singling out stdin because it's never needed by a child process
and because it's value is 0, which makes it easy to accidentally
do bad things to (commit 5c611d93)

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>