Disentangle the interaction parser from the variable parser.

Disentangle the interaction parser from the variable parser.

Both GLM and LOGISTIC REGRESSION used the interaction parser, but only GLM
used it to parse interactions in the full sense.  LOGISTIC REGRESSION was
just parsing a list of variables and then creating interactions from them.
This commit moves the interaction parser from variable-parser into GLM and
adds new code to LOGISTIC REGRESSION that satisfies its needs.