Introduce wp_get_user_contact_methods() as a public version of _wp_get_user_contactmethods.

props johnnyb.
fixes #24273.

Built from https://develop.svn.wordpress.org/trunk@25606


git-svn-id: http://core.svn.wordpress.org/trunk@25523 1a063a9b-81f0-0310-95a4-ce76da25c4cd