[Fix] Option for auto login of a specific selected user profile

According to the [Milestone 11.0](http://trac.xbmc.org/milestone/11.0)  release notes, it should be possible to select  a specific profile for auto login. However, this feature is missing.

This pull request adds this function as it is described in the patch of Trac [#10708](http://trac.xbmc.org/ticket/10708). At the moment only the first part is implemented:
"1. This patch comes in three parts. The first part adds <autologin></autologin> to profiles.xml. It defaults to a blank string (logs in the most recent user, like before). If you change this to an integer, XBMC will load that profile."

If this fix is accepted, I will add the other two patches as well (exposing this new setting to the skin and adding support for it in Confluence).

Note that the code is by bruing. I do not want to take credit for it.