Add support for keypad enter key

For now, on a fullsized keyboard, only the return key (on top of right shift) works.
Validating with the enter key (on the keypad) doesn't do anything.
I can't test on a us/uk keyboard, unfortunately, so I don't know if the fr/azerty configuration is an issue.

```
$ localectl
System Locale: LANG=fr_FR.UTF-8
VC Keymap: fr
X11 Layout: us
X11 Model: pc105+inet
X11 Options: terminate:ctrl_alt_bksp
```