exp/sql: NumInput() allow -1 to ignore checking.

Some database driver can't get number of parameters.
For example:
        http://support.microsoft.com/kb/240205/en-us
So, added way to ignore checking number of parameters with return -1.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5376091

src/pkg/exp/sql/driver/driver.go

@@ -97,6 +97,9 @@ type Stmt interface {
 	Close() error
 
 	// NumInput returns the number of placeholder parameters.
+	// -1 means the driver doesn't know how to count the number of
+	// placeholders, so we won't sanity check input here and instead let the
+	// driver deal with errors.
 	NumInput() int
 
 	// Exec executes a query that doesn't return rows, such

src/pkg/exp/sql/sql.go

@@ -474,7 +474,10 @@ func (s *Stmt) Exec(args ...interface{}) (Result, error) {
 	}
 	defer releaseConn()
 
-	if want := si.NumInput(); len(args) != want {
+	// -1 means the driver doesn't know how to count the number of
+	// placeholders, so we won't sanity check input here and instead let the
+	// driver deal with errors.
+	if want := si.NumInput(); want != -1 && len(args) != want {
 		return nil, fmt.Errorf("db: expected %d arguments, got %d", want, len(args))
 	}
 
@@ -570,7 +573,11 @@ func (s *Stmt) Query(args ...interface{}) (*Rows, error) {
 	if err != nil {
 		return nil, err
 	}
-	if len(args) != si.NumInput() {
+
+	// -1 means the driver doesn't know how to count the number of
+	// placeholders, so we won't sanity check input here and instead let the
+	// driver deal with errors.
+	if want := si.NumInput(); want != -1 && len(args) != want {
 		return nil, fmt.Errorf("db: statement expects %d inputs; got %d", si.NumInput(), len(args))
 	}
 	sargs, err := subsetTypeArgs(args)