Always use get{addr|name}info() when available

Always use get{addr|name}info() when available

Both getaddrinfo() and getnameinfo() are now used always when available, and
not only when compiling ngIRCd with support for IPv6.

This not only enables ngIRCd to handle multiple addresses per hostname when
compiled without support for IPv6, but fixes binding ngIRCd to IP addresses
on Mac OS X (and probably other BSD-based systems) as well: these systems
require that sockaddr_in is zeroed out and sockaddr_in.sin_len is set to
sizeof(sockaddr_in) like that:

  src/ipaddr/ng_ipaddr.c, line 54:

        assert(ip_str);
      + memset(addr, 0, sizeof *addr);
      + addr->sin4.sin_len = sizeof(addr->sin4);
        addr->sin4.sin_family = AF_INET;

But this would break all the systems not using sockaddr_in.sin_len, for
example Linux -- so we assume that all these systems provide getaddrinfo()
and use that for now.