power_daemon: Refactor to handle multiple exported power devices.

- Each PowerMonitor now exports a set of descriptors rather than
only a single one. This allows e.g. PowerButtonMonitor to watch

power_daemon: Refactor to handle multiple exported power devices.

- Each PowerMonitor now exports a set of descriptors rather than
only a single one. This allows e.g. PowerButtonMonitor to watch
multiple possible button event sources rather than one.

This gets power button working again on some hardware that exports
multiple ACPI power buttons, but offers no obvious way to determine
which one is actually active/being used. In the long term though, it'd
be nice to have a well-defined kernel power event interface that an app
could subscribe to, rather than having to watch individual devfs
descriptors.

Thanks to tqh and korli for advice/review.

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power_daemon: added lid_monitor.

Thanks Rene!