MDL-39249 Return to the correct page after successful update deployment

The method available_update_deployer::make_execution_widget() used to
have hard-coded return URL. Now it accepts the return URL as the second
parameter and passes it to the mdeploy.php utility.

The callerurl parameter is now correctly passed and used.
MDL-39249 Fix typo in available_update_deployer class

I have accidentally spotted this typo while working on this branch. It
has never caused any harm as both parameters callerurl and returnurl are
always passed. Blame copy/pasting of code blocks.