Add -b / --use-bit option to display rates in bit/s instead of bytes/s

Signed-off-by: Thomas Graf <tgraf@suug.ch>
curses: only initialize curses module if actually being used

Fixes display problems with plain ascii output

Signed-off-by: Thomas Graf <tgraf@suug.ch>
unit: Do not cache unit divisor list result

Now that we support multiple divisor variants caching no longer makes sense

Signed-off-by: Thomas Graf <tgraf@suug.ch>