Add `obsdfreqd` support

[morgant]

As I've noted in README.md:

Note: While I suggest using Solène Rapenne's obsdfreqd for customizing CPU performance, using apmd's automatic mode should not be used at the same time. The "apm" MenuExtra included in this project does not include obsdfreqd support, though the other MenuExtras will continue to show accurate CPU frequency/temperature, battery level, etc.

I should update "apm" MenuExtra in this project to support obsdfreqd(1). It might require re-implementing using xmenu(1), like I used for my xsndiomenu project.

[morgant]

I am working on a new libexec/xapmmenu using xmenu, based on xsndiomenu.