To make updating large attribute sets faster, the update scripts
are now run in parallel.
Please note the following changes in semantics:
- The string passed to updateScript needs to be a path to an executable file.
- The updateScript can also be a list: the tail elements will then be passed
to the head as command line arguments.
Lot of my GNOME packages have several legacy aliases,
which causes them to be updated multiple times.
This patch uses lib.unique to remove the duplicates.
