make_download_link() accepts '0' as the $label.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5320 2fea6a4d-a838-0410-917b-93a53c48e9d2
Document make_download_link().

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5321 2fea6a4d-a838-0410-917b-93a53c48e9d2
Moved make_download_link() to txplib_misc.php.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5322 2fea6a4d-a838-0410-917b-93a53c48e9d2