Get rid of warnings from childprocess's library and spec code.

Hello,

In trying to make rspec-core warning free during its spec suite, I noticed that one warning generated was from childprocess, which is a dependency of rspec's. This pull request makes childprocess's own code and specs warning free when running the specs. The remaining warning is from rspec-mocks:

```
pete@balloon:~/projects/childprocess$ rake
/home/pete/.rvm/rubies/ruby-1.9.3-p392/bin/ruby -I lib:spec -w -S rspec spec/abstract_io_spec.rb spec/childprocess_spec.rb spec/io_spec.rb spec/jruby_spec.rb spec/unix_spec.rb spec/windows_spec.rb
/home/pete/.rvm/gems/ruby-1.9.3-p392@childprocess/gems/rspec-mocks-2.13.1/lib/rspec/mocks/message_expectation.rb:148: warning: `*' interpreted as argument prefix
..............*..................

Pending:
  ChildProcess lets a detached child live on
    # how do we spec this?
    # ./spec/childprocess_spec.rb:137

Finished in 3.69 seconds
33 examples, 0 failures, 1 pending
```

Please let me know what you think, and thanks for making the specs so easy to run and understand!

See rspec/rspec-core#907