discards all of the data in the pty that hasn't been read. This was
leading to data being discarded when files were redirected into
mpirun and read by rank 0 of the job. This was very "not good".
The decision to not use ptys for stdin was made based on what Tim said
that LA-MPI was doing.
This needs to go to the v1.0 branch... Tim should probably review...
This commit was SVN r8892.