User dropdowns in wp-admin have traditionally shown the users' display names.
However, this causes ambiguity when users share display names. To correct this,
we now show the unique user_login in parentheses after the display name.
The new `display_name_with_login` value for the `show` parameter of
`wp_dropdown_users()` enables this functionality. The default value of `show`
has not been changed, for backward compatibility, but all instances of
`wp_dropdown_users()` in core wp-admin have been switched.
This changeset also reduces some duplicated logic when assembling a user list
when `include_selected` is true.
Props krogsgard, boonebgorges.
Fixes#31251.
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