Pattern analysis simplifications.

[linas]

Pattern analysis can be simplified and generalized as follows:

• Create a map of all literals contained in a given clause. This effectively creates a unified pmandatory plus absents on a per-clause basis.
• Then, clause by clause, try to ground those literals, and evaluate the clause if possible (if it is evaluatable).
• Mark present-links as evaluatable... alter PatternMatchEgine.cc to handle them directly.
• always has to be kept separate....
• Remove _fixed and use the ptm->hasAnyVirtual() to determine component bridging.

This is a blocker for #2644