Refactor kwdemo and make output translatable
Better reflect the actual behaviour of the extension:
- Make map arguments and -f/--rcfile not mutually exclusive but
extend the current configuration
- Map arguments and -f/--rcfile both override the defaults even
when -d/--default is specified
- -d/--default only overrides the current configuration
Inform the user about extending/overriding behaviour, but only at
the beginning; the following messages become terser, making the
output translatable without too much code clutter.
Rephrase help (use "short/long" option notation etc.).