use tune2fs to switch to ext3 if available after formatting new imagefile, fixes #43

It seems that the busybox mke2fs only supports formatting as ext2, and its
tune2fs does not support the -j flag for turning on the ext3 journal. So in
order to use ext3, format then convert using Debian's tune2fs. To use
ext4, it should be mountable, but it'll have to be formatted off the device
when unmounting, try to delete the loop device at the same time
on complete uninstall, remove /data/local/bin/debian
remove-debian-setup.sh: use 'set -x' to show all steps taken by script
create shortcut scripts as early as possible, since they're only created here

The shortcut scripts /debian/shell and /data/local/bin/debian are only
create by the create-debian-setup.sh script. If there is an error in the
script before those shortcuts are created, you can be left with a usable
system but without those shortcuts. Create those scripts as early as
possible to they're available if there is a working setup.
make a basic chroot to run e2fsck.static in, to provide /etc/mtab
set 'umask 077' to make all files created private to this app
make log buffer static so it can be used throughout the app
add a preference to check the SHA1 sum of the image file before starting