Fix bug #249: rstemp sets fail. Problem was that rset files were

closed when they shouldn't. Added function rfd_is_last that returns
1 if the RSFD supplied is the last one; 0 otherwise.
Added a use_list member in the RSET which is a list of RSFDs in an
RSET (opposite of free_list).  This means we can track RSFD leaks.