Previously the parameters were just dropped. Now they can be read
from within the handler script. An example to show this is added.
Makes use of the new writeShellScript function as suggested in:
issue #21557
resolves: #21557
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
