Fixes login / logout when HTTP Basic Headers are avilable.

This patch..

1. Ignores HTTP Basic Auth Headers when they are incorrect.
2. Adds a cookie 'oc_ignore_php_auth_user' when a user with successful 'BasicAuthLogin' wants to logout.
    * Fixes impossible logout with matching HTTP Basic User/Pass bug
3. Decreases the timelimit of the 'oc_ignore_php_auth_user' cookie to 5 minutes when a user who needed the cookie logs out.
    * Reallows HTTP Basic Auth for that browsing session
4.  HTTP Basic Username must match the value of the 'oc_ignore_php_auth_user' cookie for it to take affect.

----
Fixes https://github.com/owncloud/core/pull/6922 https://github.com/owncloud/core/issues/4556
In my opinion this is much cleaner and HTTP compliant than using http://whatever@domain.com as it has been depreciated in Chrome and IE.