add the module in \sn and friends when ambiguous.

[kohlhase]

I know that I said "if in doubt (about the module), leave it out". But I was wrong, ... so please change that now.

We need the module in an \sn{foo?bar} iff \sn{bar} would be ambiguous (as to which module it is from) in the current context.

I know that later annotations can introduce ambitutity and maybe we could maintain ambiguity in the current file.

I have the feeling that we are doing now introduces too many ambiguities. If all else fails, we will have to always give the module.