[krbdev.mit.edu #6335] test failures in password changing

[Ken Raeburn via RT]

Looking at the changes to network.c in adapting it for kadmind, I see the dispatch routine is 
being passed the destination (local) address from the network.c code, but note that 
recv_from_to does *not* always fill it in; it'll set *tolen to 0 if it can't get the address, and 
there's no check for that failure.

So I expect it'll fail on systems where there's no IP_PKTINFO or IPV6_PKTINFO socket option (like 
Mac OS X when using IPv4), and it'll pass on Linux (which has both).  If I tweak recv_from_to to 
pre-fill the buffer with the local IPv4 address, the test passes; if I pre-fill it with a different IPv4 
address, some debugging code I added to rd_priv.c spits out that address as the non-matching 
s_address field in the message coming back from kadmind.

See also #6205, a patch to use IP_RECVDSTADDR in kadmind...