clean up email links a bit

• core/print_api.php
  (print_user): don't print a link when the user's email is empty
  (get_email_link): process the strings before putting them in a url
  (get_email_link_with_subject): process the strings before putting them in
  a url

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1993 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9