Check for working getaddrinfo() function

At least AIX 4.3.3 and 5.1 have a broken implementation of getaddrinfo()
which doesn't handle "0" as numeric service correctly. This patch adds
a configure check for this case and changes all calling functions to only
use getaddrinfo() if it "works".

See <http://www.stacken.kth.se/lists/heimdal-discuss/2004-05/msg00059.html>
OS X has a working getaddrinfo() implementation