manager: rework generator logic
Previously generated units were always placed at the end of the search path. With this change there will be three unit dirs instead of one, to place generated entries at the beginning, in the middle and at the end of the search path: beginning: for units that need to override all configuration, regardless of user or vendor. Example use: system-update-generator uses this to temporarily redirect default.target. middle: for units that need to override vendor configuration, but not vendor configuration. Example use: /etc/fstab should override vendor supplied configuration (think /tmp), but should not override native user configuration. end: does not override anything but is available as well. Possible usage might be to convert D-Bus bus service files to native units but allowing vendor supplied native units to win.
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