Ensure wp::send_headers() detects a comments feed when permalinks are disabled and thus the withcomments QV is omitted. This fixes Last-Modified.

props sweetie089.
fixes #24622.

Built from https://develop.svn.wordpress.org/trunk@25683


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Don't override a PHP memory_limit specified using G shorthand.

props peterjaap.
fixes #23251.

Built from https://develop.svn.wordpress.org/trunk@25684


git-svn-id: http://core.svn.wordpress.org/trunk@25600 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Support G shorthand for WP_MEMORY_LIMIT. see #23251.

Built from https://develop.svn.wordpress.org/trunk@25685


git-svn-id: http://core.svn.wordpress.org/trunk@25601 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Reset "Page on Front" when such a page is trashed.

props danielbachhuber.
fixes #24171.

Built from https://develop.svn.wordpress.org/trunk@25686


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