Adding --system option to define the resulting image type to be a live system or a normal, non-live system.

This is the first step in conglomerating code for all hdd-ish
image types, independent of the partitioning scheme being used,
and adding future hdd-ish image types for virtualiziation software.

Adding support for ext4 for binary and chroot filesystem, including partial patch from Ben Howard <ben.howard@canonical.com> (Closes: #639298).

Adding kubuntu mode in order to have different defadefault hooks enabled for ubuntu and kubuntu (needed for #630088).

In order to be consistent with terminology used in debian-policy,
we've always tried to use the same names for the same things.

The repository option is one of the last things that were too
late in squeeze release cycle to be renamed in time.

Moving 'default' hooks for minimal and stripped to example hooks, they will have to be enabled manually now (Closes: #627183).

Previously, there were a couple of hooks that were automatically run
for some image flavours. This came back from the old days when we
had to deconfigure gdm and enable kdm explicitly for kde images.

After lenny, this hack was not needed anymore, and there's not much
sense to keep the mechanism in place at all. Better, let's treat all
images the same way wrt/ hooks and don't do special automagics.

The only images that is of concern of are minimal and stripped
which we don't build by default anyway. This also stopps confusing
others.

Correcting wrongly attributed -m switch to --mirror-bootstrap, thanks to Colin Watson <cjwatson@ubuntu.com> (Closes: #626905).