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.