Bug#550428: getaddrinfo: AI_CANNONAME should not do PTR lookups

[Greg Hudson <ghudson@MIT.EDU>]

Here's a recently filed upstream bug report with a test program, some
analysis of what goes wrong, and a candidate fix from me:

http://sourceware.org/bugzilla/show_bug.cgi?id=15218

Note that the bug only happens with ai_family=AF_INET (or in some cases
with ai_flags containing AI_ADDRCONFIG, I assume because it devolves
into ai_family being set to AF_INET).

Also, Fedora has had a fix for a while, which is more invasive than my
fix but obviously much more tested:

http://pkgs.fedoraproject.org/cgit/glibc.git/plain/glibc-fedora-gai-canonical.patch