Rename the new custom-background arguments passed to add_theme_support().

 * background-position becomes default-position-x
 * background-attachment becomes default-attachment
 * background-repeat becomes default-repeat

These are in line with existing default-image and default-color arguments, and also makes the position argument unambiguous and forwards compatible.

fixes #20816.

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git-svn-id: http://core.svn.wordpress.org/trunk@26484 1a063a9b-81f0-0310-95a4-ce76da25c4cd