Cull through ffi/**/*.rb and ensure string/pointer soundness

[turboladen]

There are quite a few GDAL functions that return strings that shouldn't be modified once returned to Ruby. For some of these, I've used :strptr as a return type, but after some more reading, that doesn't seem right.

In the Types page, it says:

:string should be considered to be const char * and the Ruby string must not be changed as long as it’s accessed by the library. If the string buffer shall be modified from C or Ruby side, use :pointer and FFI::MemoryPointer instead.

I think the attach_function calls should return :string, then in the wrapper functions, those values should be made frozen.