psql: Rename results to result when only a single one is meant

While reviewing code related to supporting multiple result sets in psql, 
it is always confusing that in psql many variables of type PGresult* are 
named "results" (plural), as if there could be multiple.  While it is ok 
in casual talk to consider a return from a query to be a bunch of stuff, 
this plural naming is inconsistent with how other code and the libpq API 
uses these terms.  And if we're going to get to multiple result sets 
support, I think we need to be more precise throughout the code.  The 
attached patch renames these variables and functions to singular where 
appropriate.