acpi_button: Fix several issues with multiple devices.
- On some hardware, both the fixed function FADT as well as the
  device-based power interfaces are present. In such a case we
  would fail to publish one or the other, depending on which was
  enumerated first, since we'd always attempt to publish the same
  name regardless. Now we differentiate the device name for fixed vs
  device.
- Only enable fixed function for actual fixed devices.
- Improve tracing.
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.