enter-chroot: Bind mount /run/udev on newer versions of udev

udev control directory in Chromium OS >=6092 moved from
/dev/.udev to /run/udev.

This commit makes sure it is accessible from the chroot (e.g. from
X server).

In the chroot, udev shipped with precise/wheezy (175) is fine with
just /run/udev, so there is no need to provide /dev/.udev as well.

Test: `sudo udevadm info --export-db | grep INPUT` should show some
40 lines of output.
enter-chroot: Simply bind mount /var/run/udev

Also invert the test (i.e. check for existence of /var/run/udev),
to protect against the very unfortunate case where /var/run/udev
is a symlink to /dev/.udev (update while chroot is running, started
with previous version of enter-chroot). In that case, bindmount
creates /dev/.udev, which is not a problem in itself, but would
break further chroot starts.