Move some auth_XXX strings from the central auth.php lang file to each one auth_XXX.php lang files

Since Moodle 1.8 is possible to have one lang dir for each auth plugin. That makes easy to distribute self-contained auth plugins.

But some strings in the admin->auth page are being retrieved from the central auth.php lang file. Those strings are:

[[auth_XXXXXXXtitle]] and [[auth_XXXXdescription]]

This causes that 3rd part auth plugins doesn't show the name nor the global description in Moodle (so they aren't 100% auto contained).

IMO we should move those strings from the central auth.php file to a collection of auth_XXXX.php files and look for them there.

It can be easily done in 1/2 hour of work. If so, please assign it to me and I'll do that for 1.9 (backporting to 1.8 if you consider it necessary).

Ciao