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
```
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