Fix exit code in case "basic Shannon measure"

[coldfix]

If the information expression consists of a basic Shannon information measure, the program will just tell you that. It is weird for a program to answer a trivial question with something equivalent to "This is trivial, go figure it out yourself" - and it successfully defies automation.

Instead, I'd like to handle the case "basic Shannon measures" exactly as other expressions, i.e.:

• Citip 'H(X)>=0' should exit with 0.
• Citip 'H(X)<=0' should exit with 1.

I didn't have a look at the itip1 and/or ITIP implementations in the source tree so far (just noting things to change for now), but I dearly hope this will be an easy thing to change.