Correctly log IPv6 addresses in kadmind

Define client_addr() in server_stubs.c and use it consistently in that
file and ipropd_svc.c.  In it, call getpeername() on the client socket
and use inet_ntop() on the result.  Previously we used inet_ntoa() on
the IPv4 socket address in the transport handle.

ticket: 7770
target_version: 1.12
tags: pullup