C-API: Unreachable Code in demo and test source

[CarawaySeed42]

Most demo and test source files contain "return -1" after the call to function "usage()".
The return statement is unreachable because the usage function already terminates the program
with exit code -1.
This leads to a lot of warnings that clutter the build output when compiling using cl/MSVC
with the current warning settings defined in the CMake listfiles.