Remove fixable unused-parameter warnings

I see other warnings within vhdlpp/lexor.cc caused by a flex bug
(see http://sourceforge.net/p/flex/bugs/115/), which I won't try to work around.
Spelling fixes

Mostly comments.
One user-visible string ("Evalutated to ") changed in the debug_eval_tree case.
Remove lint detected by clang

With this change, iverilog builds and runs with no extra warnings when using clang 3.0