Builtins writing to stdout don't abort when stdout has closed

Using `head` to terminate long-running command after x lines is a common idiom. Here, it's used to stop reading a terabyte log file after sufficient matches are found via `grep`:

```
> tac /hddpool/nginx.access.log | grep -i keep-alive | head -n8
# to stdout, from `grep`
/hddpool/nginx.access.log: Os { code: 32, kind: BrokenPipe, message: "Broken pipe" } # to stderr, from `tac`
```

but with `string match` instead of grep:

```
> tac /hddpool/nginx.access.log | string match -ei keep-alive | head -n8
# to stdout, from `grep`
# script hangs here w/ `tac` running but `head` aborted (currently a zombie)
```

When stdout is closed, builtins need to terminate and then exit (closing their `stdin` in the process).

I already patched all the builtin write functions to return success/failure in 3.6.0, but we're going to need to start checking the return code in each builtin as it writes to stdout so we can abort early.