multiple username selection should be allowed when using select instead of autocompletion

NancyDru has pointed out that a select has more sense (http://drupal.org/node/578488#comment-2124678),
since dblog entries are limited

in addition, having a select allows multiple username selection
currently it is a single select, which doesn't make much sense with respect to other filtering options

if a single select would be allowed then it should be optionally configurable as well as #623776: optionally offer select/autocomplete widget for username filter
then having three options would make more sense:
- single select
- multiple select
- autocomplete