Merged from trunk 8265:8270

   Merged from trunk 8265:8270

   | 8266    | Replace sys.exit() calls with return statements in           |
   | zmedico | emerge_main().                                               |

   | 8267    | Add back support for handling missing system/world atoms     |
   | zmedico | that was removed with depgraph.xcreate(). This doesn't have  |
   |         | support for mapping atoms back to the original sets yet, but |
   |         | at least a masked or unavailable system/world atom won't     |
   |         | compeletely break the depgraph now.                          |

   | 8268    | Add back support for detecting if the world file contains    |
   | zmedico | atoms that match packages in package.provided.               |

   | 8269    | Add back support for helpful `emaint --check world`          |
   | zmedico | suggestions and also show package masking reasons when       |
   |         | appropriate.                                                 |

   | 8270    | Fix broken import.                                           |
   | zmedico |                                                              |

svn path=/main/branches/prefix/; revision=8275