Create a symbolic link from /usr/local/chroots to /mnt/stateful_partition/crouton:
Makes sure that dev_install does not wipe the chroots.

Also, migrate existing chroots to new directory (transparent to the user, and very
fast as we stay on the same filesystem).
Fixes after review.
Install chroots in /mnt/stateful_partition/crouton/chroots
so we can put other stuff in /mnt/stateful_partition/crouton (e.g. shared directories).
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