(REF) CMSUser::create - Rename misleading parameter

(REF) CMSUser::create - Rename misleading parameter

The second parameter is very confusing:

* The name `$mail` suggests that the parameter is an email address.
* The parameter `$mail` actually contains the name of the *field* which has the email address.

This is a weird contract. It invites misinterpretation -- and now, e.g., some of the `Standalone`
code has misused it.

To confirm that `$mail` is actually the parameter-name, note that:

1. `create()` actually looks for `$params[$mail]`
2. Callers like `CRM/Contact/Form/Task/Useradd.php` actually give a parameter name