Support an array or comma-seperated list of excluded category IDs in...

Support an array or comma-seperated list of excluded category IDs in get_adjacent_post(). Props solarissmoke. fixes #17673

git-svn-id: https://core.svn.wordpress.org/trunk@18477 1a063a9b-81f0-0310-95a4-ce76da25c4cd