SIGINT handling on multiline PS1

Sending an interrupt signal to mrsh while having a multline prompt redraws the prompt incorrectly:

```shell
PS1="$(printf 'line1\nline2: ')" ./mrsh
line1
line2: ^C
line2:
```

One way that seems to work is to add rl_reset_line_state() before rl_replace_line() in sigint_handler.

Another hacky way is to set the readline variable `rl_event_hook` to a no-op function (e.g. `static int no_op(void) { return 0; }`) and when handling the signal interrupt (in `static void sigint_handler()`), the _only_ thing you do is set the readline variable `rl_done` to 1.