return to psub --file being the default

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

tests/psub.in

@@ -0,0 +1,49 @@
+# Test psub behavior
+set -l filename (echo foo | psub --testing)
+test -f $filename
+or echo 'psub is not a regular file' >&2
+rm $filename
+
+set -l filename (echo foo | psub --testing --file)
+test -f $filename
+or echo 'psub is not a regular file' >&2
+rm $filename
+
+set -l filename (echo foo | psub --testing --fifo)
+test -p $filename
+or echo 'psub is not a fifo' >&2
+rm $filename
+
+cat (echo foo | psub)
+cat (echo bar | psub --fifo)
+cat (echo baz | psub)
+
+set -l filename (echo foo | psub)
+if test -e $filename
+    echo 'psub file was not deleted'
+else
+    echo 'psub file was deleted'
+end
+
+# The --file flag is the default behavior.
+if count (echo foo | psub -s .cc | grep -o '\.cc$') >/dev/null
+    echo 'psub filename ends with .cc'
+else
+    echo 'psub filename does not end with .cc'
+end
+
+# Make sure we get the same result if we explicitly ask for a temp file.
+if count (echo foo | psub -f -s .cc | grep -o '\.cc$') >/dev/null
+    echo 'psub filename ends with .cc'
+else
+    echo 'psub filename does not end with .cc'
+end
+
+set -l filename (echo foo | psub -s .fish)
+if test -e (dirname $filename)
+    echo 'psub directory was not deleted'
+else
+    echo 'psub directory was deleted'
+end
+
+diff -q (__fish_print_help psub | psub) (psub -hs banana | psub)