Filter format names not properly escaped.

If filter formats contain HTML, that HTML is not properly escaped. This would theoretically be an XSS attack vector if it were not for the fact that someone with permission to 'administer filters' can already execute XSS attacks as much as they like. In a more benign version, this is a bug because it will break the page HTML if, say, a trailing < is included in the filter name. The attached patch solves the problem by check_plain()'ing the filter name. Note that the filter name was already check_plain()'d in the select list, so it's not like filters were supposed to be able to have HTML in them.