The recent change to switch `psub` to use `argparse` caused it to use
a fifo by default because it inadvertently fixed a long standing bug in
the fish script. This changes the behavior back to `psub --file` being
the default behavior and introduces a `--fifo` flag. It also updates the
documentation to make it clearer when and why `--fifo` mode should not
be used.
Fixes#4222
