Allow closing a Vagrant::Util::Subprocess's STDIN

Previously, there was no way to close the STDIN stream of a subprocess,
so commands that read from stdin in a subprocess would hang forever,
such as `/bin/sh -s`. If one tried to close the stdin, the
IO.select() call in Subprocess#execute would raise an error for calling
select() on a closed IO.

Here's a concrete example of a command that needs to close STDIN to work
properly:

```ruby
script = SOME_VERY_LONG_STRING
command = %w(ssh foo.example.com /bin/sh -s foo bar)
result = ::Vagrant::Util::Subprocess.execute(*command) do |type, data_or_io|
  if type == :stdin
    data_or_io.write(script)
    data_or_io.write("\n")
    data_or_io.close
    next
  end

  puts "Remote: #{data_or_io}"
end
```

lib/vagrant/util/subprocess.rb

@@ -144,10 +144,11 @@ module Vagrant
         # Record the start time for timeout purposes
         start_time = Time.now.to_i
 
+        open_readers = [stdout, stderr]
+        open_writers = notify_stdin ? [process.io.stdin] : []
         @logger.debug("Selecting on IO")
         while true
-          writers = notify_stdin ? [process.io.stdin] : []
-          results = ::IO.select([stdout, stderr], writers, nil, 0.1)
+          results = ::IO.select(open_readers, open_writers, nil, 0.1)
           results ||= []
           readers = results[0]
           writers = results[1]
@@ -178,8 +179,14 @@ module Vagrant
           break if process.exited?
 
           # Check the writers to see if they're ready, and notify any listeners
-          if writers && !writers.empty?
-            yield :stdin, process.io.stdin if block_given?
+          if writers && !writers.empty? && block_given?
+            yield :stdin, process.io.stdin
+
+            # if the callback closed stdin, we should remove it, because
+            # IO.select() will throw if called with a closed io.
+            if process.io.stdin.closed?
+              open_writers = []
+            end
           end
         end