Merged from trunk 10509:10521

   Merged from trunk 10509:10521

   | 10511   | Fix PackageSet.findAtomForPackage() to find the most         |
   | zmedico | specific atom since it can affect behavior when deciding     |
   |         | whether or not to add a slot atom to the world file.         |

   | 10513   | Optimize the new --skipfirst code so that in only has to     |
   | zmedico | build a new depgraph one time when there are unsatisfied     |
   |         | deps. This works by recursively traversing the digraph to    |
   |         | remove the parent packages whose deps become unsatisfied     |
   |         | when their dependencies are pruned from the mergelist.       |

   | 10514   | Clean up reference to parts of the first graph for           |
   | zmedico | --skipfirst.                                                 |

   | 10516   | * Tweak isvalidatom() to treat "null" category as valid, but |
   | zmedico | missing category as invalid (previously both where treated   |
   |         | as invalid). * Fix a spot inside emerge's unmerge() function |
   |         | that sometimes generates invalid atoms by adding an          |
   |         | unnecessary "=" to the front. Even when in cases that        |
   |         | resulted in a valid atom here, adding the "=" was redundant  |
   |         | because dep_expand() does that automatically now (for        |
   |         | backward compat).                                            |

   | 10518   | Fix TypeError: 'tuple' object does not support item          |
   | zmedico | assignment                                                   |

   | 10519   | Add some isvalidatom() test cases with "null" and missing    |
   | zmedico | category.                                                    |

   | 10521   | Fix create_world_atom() to take a Package instance in it's   |
   | zmedico | arguments.                                                   |

svn path=/main/branches/prefix/; revision=10544