Use a filter instead of recently added $format argument in get_search_form(). props WraithKenny. fixes #23850. see #15081.

git-svn-id: http://core.svn.wordpress.org/trunk@23798 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: remove HTML5 argument for search form, using a filter instead. See #23850, closes #23701.

git-svn-id: http://core.svn.wordpress.org/trunk@23799 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Rename 'get_search_form' action to 'pre_get_search_form' to prevent collision with the filter of the same name. Make sure the filtered result is not null to prevent search form from disappearing if an action function is attached to the old hook. fixes #19321.

git-svn-id: http://core.svn.wordpress.org/trunk@23800 1a063a9b-81f0-0310-95a4-ce76da25c4cd
In the HTML outputted from get_search_form(): increment the ID connecting <label> to <input> when the function is called multiple times, remove the form and submit button IDs for HTML 5.0 and for XHTML after the first call. Props WraithKenny, fixes #16539

git-svn-id: http://core.svn.wordpress.org/trunk@23801 1a063a9b-81f0-0310-95a4-ce76da25c4cd