Improve reliability of have_powershell

I have a case where on a Windows 2008 R2 host with PowerShell 2.0 the 'have_powershell' method times out.  When I interactively run the command I find that the output stops after the PowerShell command and the token from 'cmd_exec' is NOT displayed.  When I hit return the shell then processes the '&echo <randomstring>' and generates the token that 'cmd_exec' was looking for.  I tried various versions of the PowerShell command string such as 'Get-Host;Exit(0)', '$PSVErsionTable.PSVersion', and '-Command Get-Host' but was unable to change the behavior.  I found that adding 'echo. | ' simulated pressing enter and did not disrupt the results on this host or on another host where the 'have_powershell' method functioned as expected.

There may be a better solution, but this was the only one that I could find.
Update powershell.rb

To be clear, the shell that was tested with was 'windows/shell_reverse_tcp' delivered via 'exploit/windows/smb/psexec'

Additional changes required to fix regex to support the multiline output.  Also, InstanceId uses a lower case 'D' on the platforms I tested - PowerShell 2.0 on Windows 2003, Windows 7, Windows 2008 R2 as well as PowerShell 4.0 on Windows 2012 R2.

This method doesn't appear to be used anywhere in the Metasploit codebase currently.
Update powershell.rb
Update powershell.rb
Land #3390, Fix have_powershell