This adds a NixOS option for setting the CPU max and min frequencies
with `cpufreq`. The two options that have been added are:
- `powerManagement.cpufreq.max`
- `powerManagement.cpufreq.min`
It also adds an alias to the `powerManagement.cpuFreqGovernor` option as
`powerManagement.cpufreq.governor`. This updates the installer to use
the new option name. It also updates the manual with a note about
the new name.
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.
