This adds support for passing a `search_columns` argument to the user controller so that users with `list_users` caps can specify which field is being searched.
Props youknowriad, joemcgill, ntsekouras, mreishus, mamaduka.
Fixes 62596.
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git-svn-id: http://core.svn.wordpress.org/trunk@59234 1a063a9b-81f0-0310-95a4-ce76da25c4cd