Correctly log IPv6 addresses in kadmind

Define client_addr() in server_stubs.c and use it consistently in that
file and ipropd_svc.c to get the client address from a transport
handle.  In it, call getpeername() on the client socket and use
inet_ntop() on the result, instead of using inet_ntoa() on the IPv4
socket address.  Provide a log_badauth2 callback to GSSRPC, so that we
get a transport handle instead of an IPv4 socket address, and use
client_addr() within it instead of inet_ntoa().

ticket: 7770
target_version: 1.12
tags: pullup