Support Unix socket listener

[tacerus]

This removes the "port" directive in favor of "bind", and for consistency also exchanges the "PORT" variable with "BIND". This allows users to override the default TCP listener with not only a different port, but also optionally with a Unix socket, by setting a "BIND" environment variable, without having to modify puma.rb.

The default listener is additionally changed to localhost to improve security in installations without customization.

[tacerus]

I don't know how to interpret the test failure.