alter sqlx.BindNamed to implicitly use question (breaking change)

To get the old behavior, code must now rebind to the desired bindvar.
For example, if you want sqlx.DOLLAR bindvars, before your code was:

    sqlx.BindNamed(sqlx.DOLLAR, "query..", myarg)

Your new code will be:

    query, args, err := sqlx.BindNamed("query...", myarg)
    sqlx.Rebind(sqlx.DOLLAR, query)

This causes a secondary binding for non-QUESTION using databases, but
this is considered to be less important because:

 * consistently using QUESTION internally makes sqlx.In and
   sqlx.BindNamed composable
 * this function is not widely used and db.BindNamed/tx.BindNamed can
   use the internal APIs to skip the double binding
 * bindvar types are sort of 95% an internal concept

named.go

@@ -287,8 +287,8 @@ func compileNamedQuery(qs []byte, bindType int) (query string, names []string, e
 }
 
 // Bind binds a struct or a map to a query with named parameters.
-func BindNamed(bindType int, query string, arg interface{}) (string, []interface{}, error) {
-	return bindNamedMapper(bindType, query, arg, mapper())
+func BindNamed(query string, arg interface{}) (string, []interface{}, error) {
+	return bindNamedMapper(QUESTION, query, arg, mapper())
 }
 
 func bindNamedMapper(bindType int, query string, arg interface{}, m *reflectx.Mapper) (string, []interface{}, error) {

sqlx.go

@@ -279,7 +279,8 @@ func (db *DB) Unsafe() *DB {
 
 // BindNamed binds a query using the DB driver's bindvar type.
 func (db *DB) BindNamed(query string, arg interface{}) (string, []interface{}, error) {
-	return BindNamed(BindType(db.driverName), query, arg)
+	q, a, err := BindNamed(query, arg)
+	return db.Rebind(q), a, err
 }
 
 // NamedQuery using this DB.
@@ -377,7 +378,8 @@ func (tx *Tx) Unsafe() *Tx {
 
 // BindNamed binds a query within a transaction's bindvar type.
 func (tx *Tx) BindNamed(query string, arg interface{}) (string, []interface{}, error) {
-	return BindNamed(BindType(tx.driverName), query, arg)
+	q, a, err := BindNamed(query, arg)
+	return tx.Rebind(q), a, err
 }
 
 // NamedQuery within a transaction.