Add support for system-wide installation of Roundcube.

 This commit adds support for using Roundcube on multiple
servers, running on same host. This support differs from
'include_host_config' in such way, that it allows
to use different db.inc.php and different main.inc.php
and different relative on different (virtual) hosts.

 If config parameter 'use_document_root' is set, all
configuration and relative paths for logs and temp files
are resolved from $_SERVER['DOCUMENT_ROOT']. It allows
to have all settings under control of web server installations,
but have Roundcube sources only once per host, under
system package manager control.