has_privs() is strict with usernames.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5351 2fea6a4d-a838-0410-917b-93a53c48e9d2
get_safe_image_types() doesn't require its own user check.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5352 2fea6a4d-a838-0410-917b-93a53c48e9d2