Add Powershell target/Bash default and other fixes

Perl and Bash are distributed with the Windows Git installers.  With this PR, the module is then limited (for the automatic target) to:

```
msf exploit(cve_2014_9390) > set PAYLOAD cmd/unix/
set PAYLOAD cmd/unix/generic       set PAYLOAD cmd/unix/reverse_bash 
```

I'll add them back when I land.