wp_unique_post_slug() should accept `$post` parameter rather than post properties

[boonebgorges]

Breaking this ticket off from #20419.

The function signature for wp_unique_post_slug()' is currently:

function wp_unique_post_slug( $slug, $post_ID, $post_status, $post_type, $post_parent )

This is a departure from wp_unique_term_slug(), which accepts two parameters: $slug and $term (where $term is a term object). For greater consistency, we should prefer a post object in wp_unique_post_slug().

There are also some cases where the current function signature requires weird hacks, such as when generating sample permalinks. See #20419. Another argument for standardizing the signature.

Latest patch is at https://core.trac.wordpress.org/attachment/ticket/20419/20419.5.diff