Allow binding UNIX domain sockets

[spladug]

As mentioned in #58, this adds support for bound sockets being UNIX domain sockets. This is something that I needed for an application on our end and think might be generally useful. This works by extending the regex in argument parsing to accept filenames that don't have colons in them as UNIX domain sockets. The state.bind stuff is then modified to be inet or unix bind objects which abstracts out the logic of how to create/bind each type of socket properly. If binding the socket results in EADDRINUSE, we'll do the connect, stat, unlink dance to try and clean up old dead sockets. The address family is added to the child environment for each socket so that workers can know which family to bind with more easily.

Issues/questions:

• I can't get the tests to run locally without errors with or without this patch, so I'm not clear on their status. Will check with Travis once that runs. It looks like the tests are failing to install packages etc. on various versions, but where they install everything is checking out.
• There's some duplication between the command socket binding/cleanup code and the unix socket cleanup code in this patch. I wasn't clear how I could de-duplicate this and maintain the descriptive error messages.
• I'm not super fond of the bind.rb file or how that stuff is structured, if you've any alternatives I'm all ears.
• I've tested in place upgrades but there might be more cases I'm not thinking about there.

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