Add the ability to customize the shell's abort message

When starting LFE on Erjang, the abort message is still the following

```
    LFE Shell V5.10.1 (abort with ^G)
```

This change allows one to specify an abort message via an environment variable, ``LFE_ABORT_MSG``, such that the following

```
$ LFE_ABORT_MSG="(abort with ^D)" \
    erl -noshell -noinput -s lfe_boot start
```

or this:

```
$ LFE_ABORT_MSG="(abort with ^D)" \
    ./bin/lfe -pa ./ebin/
```

renders the starting shell message to be this:

```
    LFE Shell V5.10.1 (abort with ^D)
```

Those are, of course, not the VMs you'd want to change that message on ;-) This one, though, is:

```
$ LFE_ABORT_MSG="(abort with ^D)" \
    lfetool repl jlfe
```