Switches from the forking service type to simple by running haveged in
the foreground. Also restricts the execution environment a bit (these
are inspired by the Debian service file).
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.
Includes configuration option for the threshold beneath which to refill
the entropy pool - defaults to 1024 bits as this is the number used in
other distro's existing service files I looked at.
