allow manual changes to the client's internal name

Right now, the client internal name is not accessible to anybody - not even uid #1. It's automatically calculated from the client name, and we need to go through stupid hoops to generate a nice-looking internal name from the node title.

Making the internal name field accessible to users would easily resolve this issue, and the field could default to the automatically calculated uname if none has been provided.

One of the issue to consider is how this will affect the signup form, where there usually isn't a specific field for the "internal name", which is after all an obscure name from the user's perspective...