mosh: use brewed openssl as default

The openssl library bundled with OS X operation system is deprecated. It is better to link against the brewed openssl library instead of the system one.

In addition the option to link against the system one is offered.
is there any convenient way to get the running program's output from homebrew?
I plan to add such lines in the recipe

```
  test do
    system "#{bin}/mosh-server new -i 127.0.0.1 -p 60001 -v"
    system "#{bin}/mosh-client 127.0.0.1 60001"
  end
```
I need to parse the output of mosh-server to get the ``MOSH_KEY`` and the actual listening port.

And the other part is dealing with ``mosh-client`` which doesn't work with shell pipe.