Fix issue 0003050: Setting Assigned To to null causes Status to change to assigned

Also, add a new constant NO_USER to use instead of the number 0 in the code

• manage_proj_edit_page.php: use NO_USER

• core/bug_api.php
  (bug_assign): don't mark the bug assigned if the user_id given is NO_USER

• core/constant_inc.php
  (NO_USER): new constant defined as 0

• core/print_api.php
  (print_user): use NO_USER
  (print_user_with_subject): use NO_USER

• core/user_api.php
  (user_get_field): use NO_USER

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2088 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9