Allow customisation of theme mod defaults via custom-background theme support.

Give the power to theme authors to select defaults when registering
custom-background support.

Props obenland. Fixes #20816.

Built from https://develop.svn.wordpress.org/trunk@26352


git-svn-id: http://core.svn.wordpress.org/trunk@26253 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add echo arguments to wp_list_comments() and Walker_Comment, allowing non-echo usage.

props mikelittle.
fixes #10948.

Built from https://develop.svn.wordpress.org/trunk@26353


git-svn-id: http://core.svn.wordpress.org/trunk@26254 1a063a9b-81f0-0310-95a4-ce76da25c4cd