maktaba#syscall doesn't support newlines properly

The `makataba#syscall` functions seem to garble newlines you try to pass to them. For example:

``` VImL
:echo maktaba#syscall#Create(['echo', "foo\nbar"]).GetCommand()
```

```
echo 'foo\
bar'
```

I think it will work properly if we just changed it to pass newlines through unmodified inside single-quoted strings.