Add -u and -g options to febootstrap-supermin-helper

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.