nsresourced: add ability to mangle specified name if necessary

nsresourced: add ability to mangle specified name if necessary

Let's optionally mangle any passed name on the server side so that it is
useful for identifying a userns, if it isn't suitable for that
right-away. This mostly means truncating it if too long.

It's just too nasty to leave this to the client side, since they'd have
to understand the precise rules for naming userns then.

While we are at it, add full Varlink IDL comments.