This is not packaged on Debian/Ubuntu, but we require it.  Fail
with a useful error message if it is not available.

This complete rewrite of the building tools turns febootstrap
into a general purpose, cross-distro, supermin appliance only
build tool.

There is now only one program 'febootstrap' which is used to
build a supermin appliance from a list of packages.

Normal appliances are not supported.

The tools are incompatible with febootstrap 2.x (use the
febootstrap-2.x branch from git to get the old package).

We weren't expanding ext2 directories and as a result we could
hit a limit when the directory grows larger than one block.

Note that this fix only applies for creating subdirectories.
For creating files (ie. ext2fs_link) we were already doing the
right thing.

Bash automatically resets euid to uid when it executes. This means that the
effective user id of a program at the point it calls febootstrap-supermin-helper
will be lost if any part of execution chain involved bash. This in turn can
result in:

* the generation of an incorrect checksum, which contains the uid.
* the generation of supermin files with a mixture of owners

The -u and -g options allow the caller to pass in an explicit user and group to
run as. febootstrap-supermin-helper will set(u|g)id as appropriate.