care of equal-key cases, eliminating bt_firsteq().  The linear search
formerly done by bt_firsteq() took a lot of time in the case where many
equal keys appear on the same page.

Fix synopsis info thanks to Yves.Martin@ensimag.imag.fr.

per bug report from Daren Sefcik <daren@partnersdata.com>.

indeed some of 'em were missing support for more node types than that...

Add some internal cross references to select.sgml.

 Thanks to Goran Uddeborg for spotting this.

result relation ... wrong ...

a SubLink with the subplan's targetlist.  This fixes a problem seen with,
for example, a subselect that uses GROUP BY.

thereby guaranteeing wrong results from a rescanned Group node.