Refactor server lookup, new address module

Laying groundwork for non-blocking connect() and DNS resolution.  The
table.c/backendi.c should be just a data structure, and not be concerned
with DNS resolution and connecting to sockets. Pulled
open_backend_socket() code from backend.c into
handle_connection_client_hello() connection.c.

This required being able to return either: hostname, socket address, or
wildcard response when looking up the server. So introduced a new
address module to handle all IP/hostname parsing and displaying.

Removed util.c is it was only used for isnumeric() which was being used
only to validate ports, so it fit well in address.c.