Final parts of the Horde_Auth rewrite.

Final parts of the Horde_Auth rewrite.

All session related code has now been moved to horde/Core.
Horde_Auth no longer depends on horde/Core.
Authentication for each application is handled entirely by the
Horde_Core_Auth_Application driver.

Removed registry functions:
getProvider()
requireAuth()

isAuthenticated() now has a new option - 'notransparent'

Removed the recently added clearAuth() call to isAuthenticated() -
isAuthenticated() is meant solely for determining whether the given app
is authenticated.  Code to invalidate a session (for example, if a
checkExistingAuth() call fails) MUST go elsewhere.

Moved last login display to a Horde LoginTask.