plm/slurm: tweak /dev/null usage (#4489)

See the ticket for more details.

cmr=v1.8.1:reviewer=rhc:ticket=4489

This commit was SVN r31351.

The following Trac tickets were found above:
  Ticket 4489 --> https://svn.open-mpi.org/trac/ompi/ticket/4489

orte/mca/plm/slurm/plm_slurm_module.c

@@ -630,8 +630,8 @@ static int plm_slurm_start_proc(int argc, char **argv, char **env,
             free(newenv);
         }
 
-        fd = open("/dev/null", O_CREAT|O_WRONLY|O_TRUNC, 0666);
-        if (fd > 0) {
+        fd = open("/dev/null", O_CREAT|O_RDWR|O_TRUNC, 0666);
+        if (fd >= 0) {
             dup2(fd, 0);
             /* When not in debug mode and --debug-daemons was not passed,
              * tie stdout/stderr to dev null so we don't see messages from orted