* 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 4.9 branch.
Props xknown, zieladam, peterwilsoncc, whyisjake, desrosj, dd32.
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Fixes#38070, #37733 by reverting part of [38429] and using the fix in Requests.
Fixes#37992 allowing for connecting to SSL resources on ports other than 443.
Fixes#37991 by not sending default ports in the `Host:` header.
Fixes#37839 to match and decode Chunked responses correctly.
Fixes#38232 allowing a SSL connection to ignore the hostname of the certificate when verification is disabled.
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This introduces a minimum value of 1 second for timeouts passed to cURL.
Internally, cURL uses alarm() for interrupts, which accepts a second-resolution timeout. Any values lower than 1 second are instantly failed rather than being rounded upwards. While this makes the experience worse for those using asynchronous DNS lookups, there's no way to detect which DNS resolver is being used from PHP.
See #33055, #8923.
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