Add a set of helpers to turn the behaviour of mbstring.func_overload off when needed. Fixes #25259

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Switch unzip_file() over to using the mbstring.func_override helper functions. See #25259

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git-svn-id: http://core.svn.wordpress.org/trunk@25309 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Switch WP_HTTP over to using the mbstring.func_overload helper functions. This change moves the check from within the Streaming-handling function to wrap the individual request, this fixes it for both cURL and Streams and any future changes to the transports which use strlen() on binary data. See #25259 See #16057

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git-svn-id: http://core.svn.wordpress.org/trunk@25310 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Make use of the mbstring.func_overload helper functions in WP_Filesystem so byte lengths are properly determined. See #25259 Fixes #25237

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Fix the action that `set_site_transient()` fires so as not to include the private option prefix. This brings set_site_transient() back in line with it's documented behaviour, and the behaviour of all other transient functions. Fixes #25213

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