Allow postrow pm link to be modified by event

Right after the core.viewtopic_modify_post_row, the variables for postrow.contact are assigned.  For the PM link, $u_pm is used instead of $post_row['U_PM'], which is editable in the above event.  As all of the other variables that are assigned to postrow.contact are able to be modified by the event, this is the one exception.  If someone (like me) wanted to hide all user contact information without using display: none, this is the one thing holding me back.  I've fixed this with my pull.