6.29 release

Conflicts:
	CHANGELOG.txt
	modules/system/system.module

Issue @2062127 by rhm50, jlindsey15, joachim: Fix documentation of system_requirements() to say it is a hook implementation

Issue #1629994 by oadaeh, Aron Novak, yurtboy: Reference correct RFC in mail functions documentation

Issue #699604 by cam8001, Albert Volkman: Change links to form API reference to just use the file name

Issue #1012018 by Albert Volkman, mjonesdinero, yashadev, amateescu, axyjo: Fix copyright dates in copyright.txt

Issue #1145700 by jbrown, mr.baileys, joachim: harden link display on dblogoverview screen in case the link might be dependent on user input with any contrib module

Issue #183435 by TR, gregmac: make drupal_http_request() more tolerant of faulty newlines in the response headers

Issue #341588 by voxpelli, mikl, Albert Volkman, dawehner: use the right JSON MIME type in drupal_json()

Issue #260934 by catch, ShawnClark, Jody Lynn, Island Usurper, joshmiller, anrikun, roychri, pdrake, Dave Cohen, sun, plach, bjaspan: When drupal_execute()ing multiple forms with same form_id in a page request, only the first one was validated.

Issue #784864 by Niklas Fiekas, kbgordon7, rdrh555, lisarex: Link in update.php instructions was pointing to an outdated handbook page. Update it.

Issue #655048 by Gábor Hojtsy, gumanist, intuited, Albert Volkman: Plural formula information was blanked when importing a poorly-formed .po file.

Issue #800968 by JacobSingh, ksenzee, Big Z: Tabledrag.js should not use for...in to iterate over an array.