[Matthias schiffer: rebase, add a few more restrictions]

Some files have received some additional refactoring.

Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.

This package will run as invariant script after each upgrade and copy
all keys from site.conf's authorized_keys entry to
/etc/dropbear/authorized_keys.

Existing keys will be preserved.

The site.conf entry 'authorized_keys' is required (if this package is
selected) and must contain a list of strings, each representing a line
of the resulting file.