Change gateway construction to allow ignoring function pointers for now.

This addresses the issue I [mentioned](https://github.com/hermannloose/cfcss/issues/24#issuecomment-26333105) in #24.

Additionally any existing `readnone` or `readonly` attributes are removed from all instrumented functions, since they all contain at least one `store` to the global `interFunctionGSR`. Dead code elimination after instrumentation could previously remove calls to instrumented functions, resulting in signature mismatch. (no bug filed)