Merged from trunk 11041:11047

   Merged from trunk 11041:11047

   | 11042   | Ensure that that Scheduler._main_loop() doesn't return       |
   | zmedico | before the merge queue is empty.                             |

   | 11043   | Remove redundant failed_packages check from                  |
   | zmedico | Scheduler._main_loop() since it's now handled inside the     |
   |         | _schedule() call.                                            |

   | 11044   | Add async execution support for pkg_preinst and              |
   | zmedico | pkg_postinst. This allows the scheduler's poll loop to run   |
   |         | so that other parallel tasks aren't starved for output       |
   |         | handling while pkg_preinst and pkg_postinst are executing.   |

   | 11045   | * Fix doebuild so "returnpid" works for preinst and          |
   | zmedico | postinst. * Add async support for pkg_prerm and pkg_postrm.  |
   |         | * Add missing "treetype" parameter to dblink constructor     |
   |         | calls. * Fix SubProcess._wait() to check self.returncode     |
   |         | _after_ calling the scheduler.                               |

   | 11046   | Call prepare_build_dirs() prior to pkg_preinst so that       |
   | zmedico | logging works properly.                                      |

   | 11047   | Unregister from poll() objects before closing file in order  |
   | zmedico | to hopefully avoid stale events that can trigger a KeyError  |
   |         | in Scheduler._poll_loop() if the event's file descriptor no  |
   |         | longer has a registered handler.                             |

svn path=/main/branches/prefix/; revision=11048