It should handle bool value not only of "true" but also of "TRUE" and "True".

Change `<=` to `<` to avoid accessing the first character of an empty string

fixes #5074

some mandatory string keys like paths must not be empty. Add the special
default value `<none>` so the CommandLineParser can enforce this and
generate an according error message for us.

requesting a previously undeclared key is most likely an programming
error. e.g. a typo "--unused vs --unsued".
So throw in those cases.

Add an according failure testcase.

and not consider its value. treat arguments with a set default value as
present.

Opps.

Simplify the code as @SpecLad suggested.

Add the bool support to the get function

All tests passed!

Made all STL usages explicit to be able automatically find all usages of
particular class or function.

This reverts commit 94b97b7a.

when it is unrequired.