Fix broken build with v750 and cleanup some code

[antihax]

Fix build issues with latest Golang and SAP RFC SDK.

Cleaned up some code that ignored errors, still a few need refactoring.

Added build script for cross compiling Windows from Linux. Compiling CGO with libraries on windows is a bit awkward.

[CLAassistant]

All committers have signed the CLA.

[bsrdjan]

Thanks a lot @antihax !

[databus23]

This merge broke master for me on Darwin at least. It does not compile anymore:

> go test -v ./gorfc
# github.com/SAP/gorfc/gorfc [github.com/SAP/gorfc/gorfc.test]
gorfc/gorfc.go:305:2: undefined: rc
gorfc/gorfc.go:305:37: undefined: length
gorfc/gorfc.go:305:75: undefined: utf8str
gorfc/gorfc.go:305:87: undefined: utf8Size
gorfc/gorfc.go:307:5: undefined: rc
FAIL	github.com/SAP/gorfc/gorfc [build failed]
FAIL