It was only being used for (*Stmt).Exec, not Query, and not for
the same two methods on *DB.

This unifies (*Stmt).Exec's old inline code into the old
subsetArgs function, renaming it in the process (changing the
old word "subset" to "driver", mostly converted earlier)

Fixes #3640

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6258045