free Ptr{Cstring} when we need to.

[visr]

Came up in #75 (comment).
We currently never free the memory, resulting in a (probably small) memory leak.

In aftercare(::Ptr{Cstring}) we should free this at least in the case of CPLGetConfigOptions, but we should probably review all functions that return Ptr{Cstring}

See also 259a27c