Allow to customize Xephyr invocation arguments so such changes would not be overwritten after Crouton update or restore from the backup.

Fixes #431
Use /var/run/crouton to mount chroots.

Avoids the need to --make-unbindable, prevents polluting the file system, and
simplifies logic.
Properly unmount old chroots that are still mounted.

Avoids issues with upgrading.
Double-unmount for old-style chroots.
Properly handle unmounted chroots.
Use return instead of continue.
Enforce prefix existence
Remove redundant set +e
Unmount chroots before unmounting the prefix.
Fix using addtrap in tests.
Add /media tests.
Fix random background test failure.
Fix path in enter-chroot, unmount-chroot, and installer messaging.
Fix chroot setup script validity test, and delete the script on failure.
Add synclient values for Peppy and Parrot.
Merge branch 'xephyr-args' of https://github.com/ibukanov/crouton
Move x11 to xorg and make x11 a meta-target.
Fix croutoncursor on 64-bit chroots. Fixes #175.
Arch installer.
Arch: source /etc/profile in enter-chroot.