GFSH parser should validate arguments

gfsh will right now happily let you use invalid arguments and just ignore them. It would be helpful if it would error when a user provides an unsupported argument, since the user probably thinks that these arguments are supposed to do something.

Just as an example I encountered, if you try to use connect with -username instead of user, it will apparently succeed because of GEODE-2247, but actually will be using anonymous authentication since you did not pass a -user argument.