Tidy up UserSearch()

The result of UserSearch was previously not documented.
Also, the hash returned contained the ID as key and as value
it had some user fields, separated by spaces, which was not useful.

It also incorporates the changes from
http://bugs.otrs.org/show_bug.cgi?id=4723 -
"Useless substitution in UserSearch()".
Merge branch 'user4723' of https://github.com/mbeijen/otrs