<| tokenization doesn't support binary commands

Perhaps a non-bug but P9P `mk` can do this:

```
<|cmd
```

where `cmd` is a binary.

`mk` can't because the `sh` in 

```
[output, success := subprocess("sh", args, "", true)](https://github.com/dcjones/mk/blob/73d1b31466c16d0a13a220e5fad7cd8ef6d984d1/parse.go#L121)
```

should be `cmd`.

Convert `<|cmd` to `<|-c cmd` to work around this.
