rc: add options/info call to enumerate options

This also makes some fields in the Options block optional - these are
documented in rc.md

fs/fs_test.go

@@ -234,10 +234,8 @@ func TestOptionMarshalJSON(t *testing.T) {
 "Name": "case_insensitive",
 "FieldName": "",
 "Help": "",
-"Provider": "",
 "Default": false,
 "Value": true,
-"ShortOpt": "",
 "Hide": 0,
 "Required": false,
 "IsPassword": false,