WP_HTTP: When streaming to file, ensure that fwrite() suceeds and correctly writes the file to disk. Fixes #16057

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WP_Filesystem: Ensure that all files are read/written correctly by verifying the return values from fwrite() and using FTP_BINARY mode (ASCII converts line endings as per the spec). See #25237

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WP_Filesystem: Let the code breathe, add some additional whitespace between method definitions and comments.

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WP_Filesyste: SSH2 PHP Extension support: Fix a case where defining a custom FS_CHMOD_FILE / FS_CHMOD_DIR constant wouldn't take effect due to chmod() failing with a invalid path. Fixes #23589

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Upgrader: Deprecate a few mostly unused functions, wp_update_plugin(), wp_update_theme(), and, wp_update_core().
wp_update_core() was still used, as it was never updated to make use of the newer Skins. Fixes #21874

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