EXITFUNC option in Msf::Payload::Windows mixin should be an OptEnum

Verification:

```
$ ./msfconsole
msf> use exploit/multi/handler
msf> set payload windows/meterpreter/reverse_tcp
msf> show options
```

- [x] Verify EXITFUNC shows up, along with a list of accepted values

```
msf> set EXITFUNC asdasdasd
msf> run
```

- [x] Verify you get a datastore validation error

```
msf> set EXITFUNC none
msf> run
```

- [x] Verify it runs