Add optional undefined vardecl in Rule::unify_*
Add optional undefined vardecl in Rule::unify_*

Convenient for debugging

inflector/atomspace,ArvinPan/atomspace,misgeatgit/atomspace,yantrabuddhi/atomspace,misgeatgit/atomspace,AmeBel/atomspace,AmeBel/atomspace,yantrabuddhi/atomspace,ArvinPan/atomspace,rTreutlein/atomspace,rTreutlein/atomspace,yantrabuddhi/atomspace,ArvinPan/atomspace,misgeatgit/atomspace,inflector/atomspace,rTreutlein/atomspace,AmeBel/atomspace,misgeatgit/atomspace,AmeBel/atomspace,inflector/atomspace,ArvinPan/atomspace,rTreutlein/atomspace,yantrabuddhi/atomspace,yantrabuddhi/atomspace,rTreutlein/atomspace,misgeatgit/atomspace,AmeBel/atomspace,inflector/atomspace,inflector/atomspace