Allow `int` to be passed in lieu of `array`, add `append` arg to `wp_set_post_categories()`. Adds more extensive unit tests for `wp_set_post_categories()`.

Props ptahdunbar for initial patch.
Fixes #16550.


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add_theme_support( 'html5' ) now defaults to comment-list, comment-form, and search-form.

This was the implicit case in 3.6.0, modified in [25193].

see #24932.

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Clarify a database upgrade string, for 3.7.

props SergeyBiryukov.
fixes #24845.

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