database/sql: add Drivers, returning list of registered drivers

Fixes #7969.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/158950043

src/database/sql/sql.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"io"
 	"runtime"
+	"sort"
 	"sync"
 )
 
@@ -36,6 +37,16 @@ func Register(name string, driver driver.Driver) {
 	drivers[name] = driver
 }
 
+// Drivers returns a sorted list of the names of the registered drivers.
+func Drivers() []string {
+	var list []string
+	for name := range drivers {
+		list = append(list, name)
+	}
+	sort.Strings(list)
+	return list
+}
+
 // RawBytes is a byte slice that holds a reference to memory owned by
 // the database itself. After a Scan into a RawBytes, the slice is only
 // valid until the next call to Next, Scan, or Close.