zshrc: updated OS recognition functions

TODO: add more to the list (e.g. *BSDs, Solaris...)
zshrc: rewrite battery function

Now supports multiple batteries
Now indicates status (^ Charging, v Discharging, = Full or Unknown)
Now supports different functions for different OSes

TODO: add configurability (zstyle, etc.)
TODO: add support for other OSes (ATM, only Linux is supported)
zshrc: renamed allcaps variable

BATTERY becomes GRML_DISPLAY_BATTERY
PERCENT becomes GRML_BATTERY_LEVEL
zshrc: converted to UTF-8