[PATCH] slurmstped: fix dup2() error with --pty

In commit

 commit b33cd7c8
 Author: Mark A. Grondona <mgrondona@llnl.gov>
 Date:   Fri Oct 7 13:52:43 2011 -0700

a minor bug was introduced because the io_dup_stdio() call was moved
out from under the 'else' case of the empty if

 if (job->pty && (task->gtid == 0))

in task.c:exec_task() This results in the error

 dup2(stdin): Bad file descriptor

For task 0 when --pty is used, because io_dup_stdio() is
inappropriately being called after login_tty().

This patch collects the stdio setup into prepare_tty(), renames
that function prepare_stdio(), and removes the io_dup_stdio()
call from exec_task().  This removes the last io_* call from
slurmstepd/task.c, so the io.h header is removed.

I didn't see a problem with moving the io_dup_stdio() call earlier in
the task setup path, but it could probably use another set of eyes.
----------
Note: This problem was at least partly fixed with
https://github.com/SchedMD/slurm/commit/4d17300c5de7a8f146072bae8baaf773ca1fe98a