WMIC ExtAPI Improvements

With the WMI calls available in ExtAPI we no longer need to stage the powershell payload in the environment variables, we can do a single call to execute it.

Also fixes an issue in Msf::Core::Post::WMIC which called `starts_with` instead of `starts_with?`

## Verification
* Set up a remote host which allows Windows Management Instrumentation: http://www.poweradmin.com/help/enablewmi.aspx
* Target that host
* Set credentials and payload
* Go.