Default to leading and trailing wildcards for site user searches. Require...

Default to leading and trailing wildcards for site user searches. Require explicit trailing wildcard asterisk request for network user searches. Disallow leading wildcards for network user searches. Move wildcard policy up the stake, allowing more flexibility in WP_User_Query. Props SergeyBiryukov. fixes #16014

git-svn-id: https://core.svn.wordpress.org/trunk@17189 1a063a9b-81f0-0310-95a4-ce76da25c4cd