find a way to make `psub --fifo` safe from deadlock

There are actually a couple of bugs here.

The easy one is [here](http://github.com/fish-shell/fish-shell/blob/master/share/functions/psub.fish#L56) — `use_fifo` is missing a sigil and is therefore a string comparison, causing `psub` to always act as `psub -f`.

Unfortunately, it's not as simple as fixing that typo as doing so will cause a non-interruptable hang under certain circumstances. I believe it occurs when the pipe buffer is exceeded? But I'm not sure how to actually determine the pipe buffer in fish. Maybe forking another process is needed? Or... something.

Anyway, here's a (hopefully) cross-platform [test case](http://gist.github.com/g-nix/6972484/raw/rhino) (about 1.5MiB and requires Java, uses openssl to decode base64). It's a standalone wrapper for [rhino](http://github.com/mozilla/rhino). There's a big hunk of base64 in the middle of it, but the script is just:
`java -jar (echo 'BIGHUNKOFBASE64' | openssl base64 -d | psub)`

Start with share/functions/psub.fish, inspect the use_fifo check, and reproduce the hang with the linked Rhino wrapper or the 1.5 MiB Java command. Done means the typo is corrected and the --fifo path handles the large input without an uninterruptible deadlock across the supported platforms.

Written by the indexing model from the issue text.