[krbdev.mit.edu #8531] KDC/kadmind may fail to start on IPv4-only systems

[Greg Hudson via RT]

The 1.15 net-server.c code was written with the assumption that on an 
IPv4-only system, getaddrinfo(NULL, ...) will return only an IPv4 
wildcard address.  According to discussion in ticket 8530, this is not 
the case; we may get an IPv6 wildcard address as well, and binding to 
that address can give an EAFNOSUPPORT error.  (I assume the 
EAFNOSUPPORT occurs because there is no IPv6 loopback address, not 
because the kernel was confgured without IPv6 support, but I'm not 
certain.)

Specifying AI_ADDRCONFIG in hints at first seems like it might work 
around this problem, but (at least in glibc) AI_ADDRCONFIG does not 
respect loopback addresses, and we want to accept connections on 
loopback.  So using AI_ADDRCONFIG is probably not a good idea.

A better workaround is to tolerate EAFNOSUPPORT errors when setting up 
sockets, as long as we can bind to at least one address returned by 
gettadrinfo().