add read/write support for pnd override files (.ovr)

Via Config.PndOvrFile and Config.get_pnd_ovr_file() and Config.PndOvrAppFile and Config.get_pnd_ovr_app_file().

The former provides access to multiple [Application-X] sections, while the latter provides access to one only (for convenience).
tweak behavior of pnd override file support

Fix some issues unsetting variables, and .ovr will now be deleted when everything is unset.

That last behavior may cause some issues with .ovr files targetting multiple apps, but libpnd doesn't opperate at a section level nor provide a way to discover which keys are set. We'll see if a revert is coming...
add Pandora.Apps.get_pnd_direct()

provided by pnd_disco_file()