Add a note about why we will always leak RDP tmpfiles

plugins/hosts/darwin/cap/rdp.rb

@@ -9,7 +9,18 @@ module VagrantPlugins
       class RDP
         def self.rdp_client(env, rdp_info)
           config_path = self.generate_config_file(rdp_info)
+          begin
             Vagrant::Util::Subprocess.execute("open", config_path.to_s)
+          ensure
+            # Note: this technically will never get run; neither would an
+            # at_exit call. The reason is that `exec` replaces this process,
+            # effectively the same as `kill -9`. This is solely here to prove
+            # that and so that future developers do not waste a ton of time
+            # try to identify why Vagrant is leaking RDP connection files.
+            # There is a catch-22 here in that we can't delete the file before
+            # we exec, and we can't delete the file after we exec :(.
+            File.unlink(config_path) if File.file?(config_path)
+          end
         end
 
         protected
@@ -25,21 +36,24 @@ module VagrantPlugins
           }
 
           # Create the ".rdp" file
-          config_path = Pathname.new(Dir.tmpdir).join(
-            "vagrant-rdp-#{Time.now.to_i}-#{rand(10000)}.rdp")
-          config_path.open("w+") do |f|
+          t = ::Tempfile.new(["vagrant-rdp", ".rdp"]).tap do |f|
+            f.binmode
+
             opts.each do |k, v|
-              f.puts("#{k}:#{v}")
+              f.write("#{k}:#{v}")
             end
 
             if rdp_info[:extra_args]
               rdp_info[:extra_args].each do |arg|
-                f.puts("#{arg}")
-              end
+                f.write("#{arg}")
               end
             end
 
-          return config_path
+            f.fsync
+            f.close
+          end
+
+          return t.path
         end
       end
     end