[krbdev.mit.edu #7132] Reverse DNS happens despite rdns=false

[Greg Hudson via RT]

This is a gnu libc bug.  AI_CANONNAME is not supposed to do a PTR lookup, 
and in fact doesn't do so in most circumstances.  The trunk code works 
around this bug as of May 3 (issue #7124); we thought we had worked around 
the bug in 1.10 (issue #6922) but it turns out that AI_ADDRCONFIG triggers 
the bug as does AF_INET4.

The suggested patch turns off too much canonicalization.  The desired 
behavior with rdns=false is that the search domain is appended and CNAMES 
are resolved, but no PTR lookup is performed.