LDAP backend to support paged results

[blizzz]

Atm, getUsers and getGroups need to fetch all result rows with a single request. This has some disadvantages, especially a fixed limiit of returned results makes problems. It is possible to fetch the result rows "page by page" with a defined number of result entries per page. However, this requires PHP >= 5.4, as the required functions 'ldap_control_paged_result_response' and 'ldap_control_paged_result' have been introduced only then. Since ownCloud should be compatible to PHP 5.3 the current methods need be retained as fallback solution.

Especially the Sharing dialoig should profit from it, as it already looks for a limited number of results.

The Admin → Users page could benefit, too, given that it will be improved to not show all users from the very beginning (ideas for this: paged results or infinite scrolling, plus filtering).

A side effect is that the search for users is being shifted away from owncloud names to LDAP attributes, i.e. also to LDAP server side. It could lead to some wondering users, because the generated ownCloud name (out of a specified attribute) is immutable – contrary to the LDAP values.
On the other hand it offers better flexibility as mutliple attributes can be included in the search (username, real name, email, ...).