orterun.c: set an fd to be close-on-exec

Make sure the debugger attach fifo is marked as close-on-exec so that
children procs don't inherit it.  For example, if you salloc a SLURM
allocation and run "mpirun ..." in there (i.e., mpirun is running on
the head node, and launching on to back-end nodes), the forked srun's
will inherit this fd if it is still open.

Refs #4550