strengthen type constraints
Tighetn the allowed variable values
potentially allow self-grounding of variables.

These are a set of changes pertaining to the discussion in bug #1092
If the NO_SELF_GROUNDING token is not set, then the self-grounding of
variables should work.  I think. Its not well-tested, and could lead
to trouble.
Merge pull request #1100 from linas/freaky

Preparation for allowing the self-grounding of variables