sshfs_forward_mount: best effort for uninheriting

In 6f285cd we made it so that we would uninherit all filehandles by
default on windows. Some users have reported that this operation
is erroring because `The parameter is incorrect.`. See #52

In this commit I am making the uninheriting operation best effort.
The rationale is that if a file handle was not able to be set to
uninheritable then it probably wasn't one that would get inherited
in the first place. The code will now print out a warning, but
continue on.

lib/vagrant-sshfs/cap/guest/linux/sshfs_forward_mount.rb

@@ -106,7 +106,7 @@ def self.sshfs_forward_unmount_folder(machine, opts)
 
         protected
 
-        def self.windows_uninherit_handles
+        def self.windows_uninherit_handles(machine)
           # For win32-process Process.create, if we pass any file handles to the 
           # underlying process for stdin/stdout/stderr then all file handles are 
           # inherited by default. We'll explicitly go through and set all Handles
@@ -115,12 +115,27 @@ def self.windows_uninherit_handles
           # https://github.com/djberg96/win32-process/blob/6b380f450aebb69d44bb7accd958ecb6b9e1d246/lib/win32/process.rb#L445-L447
           # bInheritHandles from https://msdn.microsoft.com/en-us/library/windows/desktop/ms682425(v=vs.85).aspx
           # 
+          # In 6f285cd we made it so that we would uninherit all filehandles by
+          # default on windows. Some users have reported that this operation
+          # is erroring because `The parameter is incorrect.`. See #52
+          # We will make the uninheriting operation best effort. The rationale 
+          # is that if a file handle was not able to be set to uninheritable 
+          # then it probably wasn't one that would get inherited in the first place.
+          #
           # For each open IO object 
           ObjectSpace.each_object(IO) do |io|
             if !io.closed?
               fileno = io.fileno 
               @@logger.debug("Setting file handle #{fileno} to not be inherited")
-              self.windows_uninherit_handle(fileno)
+              begin
+                self.windows_uninherit_handle(fileno)
+              rescue SystemCallError => error
+                msg = "Warning: couldn't set file handle #{fileno} to not be inherited\n"
+                msg+= "Message: " + error.message + "\n"
+                msg+= "Continuing in best effort...."
+                machine.ui.warn(msg)
+                @@logger.warn(msg)
+              end
             end
           end
         end
@@ -226,7 +241,7 @@ def self.sshfs_slave_mount(machine, opts, hostpath, expanded_guest_path)
             # For windows we need to set it so not all file handles are inherited
             # by default. See https://github.com/dustymabe/vagrant-sshfs/issues/41
             # The r1,r2,w1,w2,f1,f2 we pass below will get set back to be shared
-            self.windows_uninherit_handles
+            self.windows_uninherit_handles(machine)
             # For windows, we are using win32-process' Process.create because ruby
             # doesn't properly detach processes. See https://github.com/dustymabe/vagrant-sshfs/issues/31
             Process.create(:command_line => sftp_server_cmd,