Add support for allowed/denied IP/Mask

Login can be restricted to specific ranges of IP address or to a specific IP
address.

Please apply the appropriate SQL upgrade script to add the filter field to your
database.

The filter database field will allow to add other filters without requiring a
new database migration

sftpd/internal_test.go

@@ -174,7 +174,7 @@ func TestUploadFiles(t *testing.T) {
 func TestWithInvalidHome(t *testing.T) {
 	u := dataprovider.User{}
 	u.HomeDir = "home_rel_path"
-	_, err := loginUser(u, "password")
+	_, err := loginUser(u, "password", "")
 	if err == nil {
 		t.Errorf("login a user with an invalid home_dir must fail")
 	}