Cleanups and support code for importing smart battery support.

• Use ACPI_BATT_UNKNOWN instead of constants
• Use maxunit instead of a count of devices since we may have sparse battery devices in the future. Only userland should be using unit numbers anyway, so provide a translation function. (Kernel use of batteries should be restricted to looking up a device_t and calling methods directly.
• Don't check acpi_BatteryIsPresent() in acpi_battery. Leave it up to the hardware-specific driver (i.e. cmbat) since smart batteries seem to not report the "battery present" flag.
• Convert mA to mW if the battery uses those units. CM-batteries only used mW so this deficiency went unnoticed.
• Clean strings reported in the battery info from any control chars.
• Only dereference the unit from ioctl_arg if the full struct is present. Unit wouldn't have been used later if it wasn't present but this is cleaner. Translate the unit if it's not ACPI_BATTERY_ALL_UNITS.
• bzero structs before returning them to usermode for future compat.

Most of this work was submitted by Hans Petter Selasky and then majorly
reworked by myself.

Submitted by: Hans Petter Selasky <hselasky / c2i.net>