Change check_output to check_markup

We have two functions for cleaning user submitted data: check_plain() for data with no markup and check_output() for HTML. For the sake of consistency, let's rename check_output to check_markup.

Contrib modules affected:

admnotify/admnotify.module
atom/atom.module
bookreview/bookreview.module
discography/discography.module
feature/feature.module
filestore2/filestore2.module
filtercache/README.txt
filtercache/filtercache-drupal44.patch
filtercache/filtercache.module
flexinode/field_textarea.inc
groups/groups.module
image/image.module
img_assist/CHANGELOG.txt
listhandler/listhandler.module
mail_archive/mail_archive.module
menu_otf/CHANGELOG.txt
naggregator/naggregator.module
node_aggregator/feed.module
node_aggregator/item.module
nodewords/nodewords.module
notify/notify.inc
og/og.module
privatemsg/privatemsg.module
project/comment.inc
project/mail.inc
project/project.module
quotes/quotes.module
recipe/recipe.module
shoutbox/shoutbox.module
sidebar/sidebar.module
sidecontent/sidecontent.module
taxonomy_html/taxonomy_html.module
tec/tec.module
trackback/trackback.module
trip_search/trip_search.module
wallpaper/wallpaper.module
webform/webform.module
moviereview/moviereview.module
question/question.module
quiz/multichoice.module