Implement battery functionality for NetBSD users

Implement battery functionality for NetBSD users

This patch takes a similar approach as the NetBSD CPU temperature
code in that it uses proplib(3) to walk dictionaries supplied by
envsys(4).

In addition to providing the basic functionality, it:

* Provides all existing format specifiers (%emptytime %consumption
  %status %percentage %remaining)
* Respects all existing config options (hide_seconds, low_threshold,
  integer_battery_capacity, last_full_capacity)
* Projects "time until full" when battery status is CS_CHARGING