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.
Revert "enter-chroot: Simply bind mount /var/run/udev" (+ invert test)

This reverts commit 24c160cee85d781d74c0018936936091a64337ad.

Bind mounting in /var/run is really a bad idea: we need to
hardcode the path in installer/prepare/ubuntu, or run another
find/xargs expression.

Bind mounting to /var/host is simpler, and works just fine.
Merge pull request #967 from drinkcat/fix-udev-control-dir-revert

Revert "enter-chroot: Simply bind mount /var/run/udev" (+ invert test)
Merge branch 'chrootdir' of git://github.com/drinkcat/chroagh into chrootdir
Cleanup of chroot directory migration output.

The case where a chroot is mounted is common, so the installer now instructs and
waits for the user to close any open chroots.

mv -T will clobber empty directories and prevent strange things from happening
if the user does something in parallel.
Don't end the line, to show that it's waiting.
Add note about pressing ctrl-C
Fix warning message output
Don't assume link exists when canonicalizing CHROOTS

Fixes #968
installer: Allow host-bin restoration only.
mkpart.sh: Create a separate partition for crouton.

Prevents accidental wiping when switching between normal and dev modes.
installer, host-bin: Automatically mount crouton partition.
mkpart.sh: Warn if ChrUbuntu is installed. Increase logout timeout.

Thanks @DennisLfromGA for testing
mkpart.sh: Check if chroots are mounted before waiting 10 seconds.

Also readlink /var/run/crouton (on most recent Chromium OS we use
/run/crouton)
We are crouton Authors...

...crunchy and yummy!