* XML-RPC: Improve error messages for unprivileged users.
* External Libraries: Disable deserialization in Requests_Utility_FilteredIterator
* Embeds: Disable embeds on deactivated Multisite sites.
* Coding standards: Modify escaping functions to avoid potential false positives.
* XML-RPC: Return error message if attachment ID is incorrect.
* Upgrade/install: Improve logic check when determining installation status.
* Meta: Sanitize meta key before checking protection status.
* Themes: Ensure that only privileged users can set a background image when a theme is using the deprecated custom background page.
Brings the changes from [49380,49382-49388] to the 5.1 branch.
Props xknown, zieladam, peterwilsoncc, whyisjake, desrosj, dd32.
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