Add with pkgCacheGen::Essential a way to control which packages get the

Add with pkgCacheGen::Essential a way to control which packages get the
essential flag:
* native is the default and will only mark packages of the main arch
* all will mark all packages which have these flag in Packages
* none will obviously do the opposite
* installed will only mark packages which are installed as essential,
  so it will behave in the same way as dpkg does it.
It is mostly needed sometimes for debugging but some users with special
needs might like to switch the mode as well under the expense to be
on their own…