Add underpinnings of UNIX domain socket support

Add underpinnings of UNIX domain socket support

Add sa2sun() and ss2sun() helpers to socket-utils.h.  Add UNIX domain
socket support to sa_socklen() and print_addr().  Expand buffers for
printing addresses to 128 bytes to accomodate the maximum UNIX domain
socket path length.

Add loop_add_unix_socket() to net-server.c, primarily using the
existing TCP support (renamed to "stream").

As there is no standard Kerberos address type for UNIX domain sockets,
add basic directional address support.  Add a definition for
ADDRTYPE_DIRECTIONAL in krb5.h.  Add private constant krb5_address
objects to libkrb5 for initiator and acceptor directional addresses.
Use directional addresses for the KRB-SAFE/KRB-PRIV source address in
the kprop and password change protocols when the transport is not IPv4
or IPv6.

krb5_address objects are used for auditing purposes in the KDC audit
and KDB pluggable interfaces.  Add a local-use address type
ADDRTYPE_UNIXSOCK for use in these cases.  Add a flag to
k5_sockaddr_to_address() to indicate whether this address type can be
used.  Add UNIX domains socket conversion support to the test audit
plugin module.

[ghudson@mit.edu: combined several commits; used directional addresses
for KRB-SAFE/KRB-PRIV; reduced duplication in net-server.c support;
wrote commit message.  Also based on work by Alexander Bokovoy.]

ticket: 9155