Spawn the payload as a seperate process

Running the payload using system() in a thread was causing some weird
interactions with ctrl-c. Fix those issues by using Process.spawn and
Process.detach. I suspect this was the original cause of rapid7#3631, java
meterpreter sessions dying unaccountably.

See rapid7#3631

Author: egypt

test/modules/exploits/test/java_tester.rb

@@ -50,7 +50,8 @@ def exploit
 			fd.write(@jar_data)
 		end
 
-		framework.threads.spawn("Module(#{self.refname})-JavaLauncher", false) { system("java -jar payload.jar") }
+		pid = Process.spawn("java -jar payload.jar &")
+		Process.detach pid
 	end
 
 end