CLEANUP: wurfl: reduce exposure in the rest of the code

CLEANUP: wurfl: reduce exposure in the rest of the code

The only reason wurfl/wurfl.h was needed outside of wurfl.c was to expose
wurfl_handle which is a pointer to a structure, referenced by global.h.
By just storing a void* there instead, we can confine all wurfl code to
wurfl.c, which is really nice.