database/sql: treat pointers as nullable types like encoding/json

database/sql: treat pointers as nullable types like encoding/json

- convert from nil pointers to the nil interface{}
- dereference non-nil pointers
- convert from nil interface{}s to nil pointers
- allocate pointers for non-nil interface{}s
- tests for all of the above

R=golang-dev, bradfitz, rsc, rogpeppe
CC=golang-dev
https://golang.org/cl/5630052