unmount everything that's mounted into the chroot

Installing certain packages may actually mount things inside the chroot,
e.g. /proc/sys/fs/binfmt_misc. This needs to be taken care of when
removing the chroot.

Add to Chroot two methods get_mounted() and unmount_all() that replace
unmount_proc(). Use them for cleanup and add some extra checking before
running rmtree().

TODO: Cooperation + order with unmount_selinux() is unclear.
TODO: LVM and VirtServ support?

TODO: Possible problems: symlinks on the way to the tempdir may cause
/proc/mounts to use a different directory name from Chroot.name, so that
prefix matching does not work and the mount points are not found.
Assuming "everything unmounted" in that case may be problematic.

A rather safe approach to cleanup the chroot would be
* umount known extra mounts (e.g. /proc/sys/fs/binfmt_misc)
* umount all known mountpoints (/proc, bindmounts)
* umount everything recorded in /proc/mounts matching this chroot
* use rm -rf --one-file-system

Signed-off-by: Andreas Beckmann <debian@abeckmann.de>