Add a File.open method for meterpreter

Same semantics as Ruby stdlib File.open - if you give it a block, yields
a new File object and closes it after the block, otherwise same as
File.new.

[FixRM #6481]

lib/rex/post/meterpreter/extensions/stdapi/fs/file.rb

@@ -290,6 +290,25 @@ class File < Rex::Post::Meterpreter::Extensions::Stdapi::Fs::IO
 		end
 	end
 
+	#
+	# With no associated block, File.open is a synonym for ::new. If the optional
+	# code block is given, it will be passed the opened file as an argument, and
+	# the File object will automatically be closed when the block terminates. In
+	# this instance, File.open returns the value of the block.
+	#
+	# (doc stolen from http://www.ruby-doc.org/core-1.9.3/File.html#method-c-open)
+	#
+	def File.open(name, mode="r", perms=0)
+		f = new(name, mode, perms)
+		if block_given?
+			ret = yield f
+			f.close
+			return ret
+		else
+			return f
+		end
+	end
+
 	##
 	#
 	# Constructor

test/modules/post/test/meterpreter.rb

@@ -199,16 +199,16 @@ class Metasploit4 < Msf::Post
 
 		it "should create and remove files" do
 			res = true
-			fd = session.fs.file.new("meterpreter-test", "wb")
-			fd.write("test")
-			fd.close
+			res &&= session.fs.file.open("meterpreter-test", "wb") { |fd|
+				fd.write("test")
+			}
 
 			vprint_status("Wrote to meterpreter-test, checking contents")
-			fd = session.fs.file.new("meterpreter-test", "rb")
-			contents = fd.read
-			vprint_status("Wrote #{contents}")
-			res &&= (contents == "test")
-			fd.close
+			res &&= session.fs.file.open("meterpreter-test", "rb") { |fd|
+				contents = fd.read
+				vprint_status("Wrote #{contents}")
+				(contents == "test")
+			}
 
 			session.fs.file.rm("meterpreter-test")
 			res &&= !session.fs.dir.entries.include?("meterpreter-test")