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

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

This makes the naming more consistent with the libpq API and the rest
of the code, and makes actually supporting multiple result sets in the
future less confusing.

Discussion: https://www.postgresql.org/message-id/flat/db72fb98-9b43-d776-7247-6ed38f28e7c6%40enterprisedb.com