IPv6 address parsing + IPv6 lookup for FastCGI and reverse proxy

This pull request contains two commits to improve IPv6 support for communication between Yaws and FastCGI backends/reverse proxies:

1. Support IPv6 address in reverse proxy URLs and as FastCGI server names, using the `[...]` syntax.
2. Allow the user to choose how FastCGI or reverse proxy server names are resolved and connections are made by setting the list of address families to try (e.g. `[inet6, inet]` if he wants IPv6 first; `[inet]` remains the default to keep the current behavior).

We need this to play with HHVM: on Linux, we set the `net.ipv6.bindv6only` sysctl to `1` and, in this case, HHVM only listen on `::1`, not `127.0.0.1`. It appears to not be configurable.