on filtering the share box users and groups whose name begins with the s...

...earch term shall appear on top, fixes #6430

![sharebox](https://f.cloud.github.com/assets/2184312/1920043/72b58226-7de0-11e3-856e-8fa0593ac01f.png)

(You need another backend like LDAP to test this properly, for local users and groups only the start of the name is matched against the search string).

Please review @schiesbn @PVince81 @bantu or others