[horde] autodetect webroot

[Per olof Ljungmark]

Upgrading from Horde 3.1.3 to 3.1.4

I had to manually enter
'webroot' => '',
in registry.php to get it to work using
http://webmail.example.com
instead of
http://webmail.example.com/horde
with all necessary changes applied to httpd.conf and config/conf.php

Is autodetect supposed to work? Could not find any bugs relating to it.