Merged from trunk -r14750:14767

   Merged from trunk -r14750:14767

   | 14751   | Bug #259697 - Update IUSE.invalid docs for GLEP 56.          |
   | zmedico |                                                              |

   | 14753   | Bug #291271 - Make vardbapi._bump_mtime() create category    |
   | zmedico | directories when necessary.                                  |

   | 14754   | Make dblink.delete() remove empty parent category            |
   | zmedico | directories.                                                 |

   | 14755   | Make cacheddir() always behave like EmptyOnError is True,    |
   | zmedico | since listdir() doesn't handle a None return value           |
   |         | correctly.                                                   |

   | 14760   | Bug #290921 - Always exit unsuccessfully if -r is not        |
   | zmedico | specified and a directory is skipped. This reverts a         |
   |         | behavior change from r9484.                                  |

   | 14761   | Allow unmerge of packages from the "selected" set when a     |
   | zmedico | package set such as @unavailable is given on the command     |
   |         | line. Thanks to Vlastimil Babka <caster@g.o> for reporting.  |

   | 14762   | For compatibility, still exit successfully if there are      |
   | zmedico | skipped directories. We can make it more strict later.       |

   | 14763   | Add a --backtrack=COUNT option to control how many times     |
   | zmedico | backtracking is allowed, and reduce the default from 30 to   |
   |         | 5.                                                           |

   | 14767   | Fix --backtrack option handling.                             |
   | zmedico |                                                              |

svn path=/main/branches/prefix/; revision=14771