Fix unwanted redirect to IdP from locallogin.php in case alternateloginurl is set, resolves #775

[krostas1983]

As detailed in the corresponding issue, this fix solves various minor problems regarding locallogin.php:

• Login form should still be displayed if local login is not available due to alternateloginurl being set.
• Redirect should be to /index.php instead of /login/index.php to prevent certain configurations (again, alternateloginurl) from redirecting to an external login prompt with a valid session.

The feature provided by locallogin.php is very helpful for those cases already where the Moodle WAYF service is not used and all users are expected to come with a valid SSO session.

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[krostas1983]

Commit is compatible with branches MOODLE_401_STABLE to MOODLE_405_STABLE.