Redesign sources.list handling. Get rid of /etc/grml/fai/apt/sources.list.

Main changes:

* no longer recursively deploy /etc/apt via fcopy
* /etc/grml/fai/apt/sources.list is gone
* /etc/apt/sources.list will no longer use entries from
  GRML_LIVE_SOURCES once the system has been initially installed

More details:

As we get rid of /etc/grml/fai/apt/sources.list we finally manage
to no longer magically touch grml-live's configuration files WRT
sources.list handling.

Instead the set up of sources.list happens as follows:

${GRML_FAI_CONFIG}/config/files/etc/apt/sources.list/GRML_LIVE_SOURCES_LIST
is generated based on $GRML_LIVE_SOURCES (iff set by the user,
otherwise it falls back to sane defaults using grml repos and the
according Debian suite). This GRML_LIVE_SOURCES_LIST file then
gets deployed on *fresh* deployments. It won't be reused anymore
for the existing installation, so if you run grml-live with the
"-u" option the system will keep /etc/apt/sources.list as it is
as well as the files inside /etc/apt/sources.list.d/. This
provides more flexibility combined with a more transparent and
consistent behaviour for the user.

Then /etc/apt/sources.list.d/debian.list will be set up based on
the detected Debian suite and the files from
${GRML_FAI_CONFIG}/config/files/etc/apt/sources.list.d/debian.list/DEBIAN_*.

Finally /etc/apt/sources.list/grml-stable.list will be deployed
and depending on the build type also
/etc/apt/sources.list.d/grml-testing.list might be installed.