Make asm spacing easier to read

Also adds a #prepends callback to Payload::Windows to make it a little
clearer what's happening.

Author: egypt

lib/msf/core/exploit.rb

@@ -1242,15 +1242,6 @@ def handler_enabled?
 		not datastore['DisablePayloadHandler']
 	end
 
-	#
-	# Returns the state of the PrependMigrate option
-	# See https://github.com/rapid7/metasploit-framework/pull/917
-	# for discussion.
-	#
-	def prepend_migrate?
-		!!(datastore['PrependMigrate'] && datastore['PrependMigrate'].to_s.downcase == 'true')
-	end
-
 	##
 	#
 	# Handler interaction

lib/msf/core/payload/windows.rb

@@ -12,7 +12,7 @@
 ###
 module Msf::Payload::Windows
 
-	include Msf::Payload::PrependMigrate
+	include Msf::Payload::Windows::PrependMigrate
 
 	#
 	# ROR hash associations for some of the exit technique routines.
@@ -25,6 +25,18 @@ module Msf::Payload::Windows
 			'none'    => 0x5DE2C5AA, # GetLastError
 		}
 
+
+	# @abstract Override to add additional stubs to prepend to the final
+	#   shellcode. Be sure to call super so other modules may add stubs.
+	# @return [String] Stub to place at the begginning of generated shellcode
+	def prepends
+		""
+	end
+
+	def generate(*args)
+		return prepends + super
+	end
+
 	#
 	# This mixin is chained within payloads that target the Windows platform.
 	# It provides special variable substitution for things like EXITFUNC and