make optional fields possible for input framework.

This do not have to be present in the input file and are marked as &optional in the record description.

Those can e.g. be used to create field values on the file in a predicate while reading a file - example:

	Input::add_table([$source="input.log", $name="input", $idx=Idx, $val=Val, $destination=servers,
				$pred(typ: Input::Event, left: Idx, right: Val) = { right$notb = !right$b; return T; }

testing/btest/Baseline/scripts.base.frameworks.input.optional/out

@@ -0,0 +1,9 @@
+{
+[2] = [b=T, notb=F],
+[4] = [b=F, notb=T],
+[6] = [b=F, notb=T],
+[7] = [b=T, notb=F],
+[1] = [b=T, notb=F],
+[5] = [b=F, notb=T],
+[3] = [b=F, notb=T]
+}