libbuxton: Set strings to const for the API

Because clients are often using "const char *" instead
of just "char *" in their internal API, it make sense
to offer this new API. Their are two main reasons:
- passing a "char*" for a "const char*" is safe
- the developer doesn't have to ask himself if removing
the const with a cast is okay or not

This change will bump our library version.
shared: Take commandline configuration option as const
libbuxton: Take set_value argument as const