WP_HTTP: Revert r30491 which updated the bundled root certificates. There's a report that this is breaking under certain PHP/OpenSSL versions (which we've encountered before), and we're safer with a slighty out of date CA bundle than breaking HTTPS communication on affected sites.
See #30434

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In the `links_add_base()` callback, use a HTTP-aware URL joining function, rather than a filesystem joining function to avoid PHP warnings on certain systems.
Fixes #29950

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git-svn-id: http://core.svn.wordpress.org/trunk@30756 1a063a9b-81f0-0310-95a4-ce76da25c4cd