util.prosodyctl.shell: Close state on exit to fix saving shell history

This ensures a last round of garbage collection and finalizers, which
should include flushing the readline history file.

Test procedure:
```
$ ./prosodyctl shell
prosody> s2s:show() -- any command that is not the last in history
... output
prosody> bye
$ ./prosodyctl shell
prosody> ^P
```

After this, the shell prompt should contain the last command from before
the "bye". Before this patch, recent history is gone most of the time.

0 files changed, 0 insertions, 0 deletions