bug#643: 23.0.60; battery.el needs deprecated ACPI_PROCFS_POWER interface

[Sven Joachim]

In recent Linux kernels, the information about battery status has been
moved from /proc/acpi/battery to /sys/class/power_supply.  It is only
available from the former place if the kernel has been built with the
ACPI_PROCFS_POWER option, which is deprecated:

,----[ /usr/src/linux-2.6.26.1/drivers/acpi/Kconfig ]
| config ACPI_PROCFS_POWER
|       bool "Deprecated power /proc/acpi directories"
|       depends on PROC_FS
|       default y
|       ---help---
|         For backwards compatibility, this option allows
|           deprecated power /proc/acpi/ directories to exist, even when
|           they have been replaced by functions in /sys.
|           The deprecated directories (and their replacements) include:
|         /proc/acpi/battery/* (/sys/class/power_supply/*)
|         /proc/acpi/ac_adapter/* (sys/class/power_supply/*)
|         This option has no effect on /proc/acpi/ directories
|         and functions, which do not yet exist in /sys
| 
|         Say N to delete power /proc/acpi/ directories that have moved to /sys/
| 
`----

At least Debian ships kernels that are built without ACPI_PROCFS_POWER,
so battery.el will not work with them.

Sven