Merged from trunk 8259:8265

   Merged from trunk 8259:8265

   | 8260    | Fix logic for the "selective" depgraph parameter so that it  |
   | zmedico | behaves correctly in cases where installed packages provide  |
   |         | old-style virtuals but none of the available packages do.    |

   | 8261    | Match packages against arguments earlier in                  |
   | zmedico | depgraph._select_package() so that corrupt PROVIDE is        |
   |         | detected sooner.                                             |

   | 8262    | Fix selective logic to work properly with masked packages.   |
   | zmedico |                                                              |

   | 8263    | remove 'system' and 'world' as actions, and remove most of   |
   | genone  | the special case code as well                                |

   | 8264    | Make world atoms non-greedy for SLOTs in action_depclean()   |
   | zmedico | since users can simply add SLOT atoms to world for any       |
   |         | specific SLOTs that they want to keep.                       |

   | 8265    | Remove --update greedy SLOTs code from                       |
   | zmedico | depgraph.select_files() so that any desired SLOTs have to be |
   |         | pulled in explicitly. Any SLOTs that are not explicitly      |
   |         | pulled in will be removed by --depclean.                     |

svn path=/main/branches/prefix/; revision=8274