tcp comstack: change behavior for @-specs (anyaddr)

@ listens on AF_INET (IPV4) peers, and @6 listens to AF_INET6 (ipv6)
peers. Previously @ listened on AF_INET6 which accepted both IPV4 and
IPV6 peers on some Linux systems. However, in general that does not
work so YAZ can not rely on that to work. Debian changed behavior in
this respect from Debian lenny to Debian squeeze (still testing as of
Oct 2010). Refer to:
http://www.mail-archive.com/debian-devel@lists.debian.org/msg277726.html