Finish repairing 6.5's problems with r-tree indexes: create appropriate

Finish repairing 6.5's problems with r-tree indexes: create appropriate
selectivity functions and make the r-tree operators use them.  The
estimation functions themselves are just stubs, unfortunately, but
perhaps someday someone will make them compute realistic estimates.
Change pg_am so that the optimizer can reliably tell the difference
between ordered and unordered indexes --- before it would think that
an r-tree index can be scanned in '<<' order, which is not right AFAIK.
Repair broken negator links for network_sup and related ops.
Initdb forced.  This might be my last initdb force for 7.0 ... hope so
anyway ...