Refactor nfs options

This is a redo of #15 with
- https://github.com/kobliha/yast-nfs-client/commit/55f2f5b4352f0b1dd614b111407b8e0e571d3f0b split into 2 commits to better see how the option list changed
- https://github.com/kobliha/yast-nfs-client/commit/a3d5395c6a4d16d862e544e36cb32b2bd9c33d2a changed to use the actual options reported in the bug
- fixed gitignore to pass`rake osc:build`