init: use SELinux /dev/null if available

SELinux provides it's own /dev/null character device at
/sys/fs/selinux/null. This character device is exactly the same
as /dev/null, including the same major/minor numbers, and can
be used wherever /dev/null is used.

Use /sys/fs/selinux/null instead of trying to create our own
/dev/__null__ device. This moves us one step closer to eliminating
all uses of mknod() by init.

/sys/fs/selinux/null is only available once the /sys/fs/selinux filesystem
is mounted. It's not available to the first stage init, so we
still have to fall back to mknod then.

Change-Id: Ic733767ea6220a130537de33cc478ae79578ce20