Add support for UNIX domain sockets

Add support for UNIX domain sockets

Make the KDC and kadmind listen on UNIX domain sockets if any are
listed in kdc_listen, kadmind_listen, or kpasswd_listen.  Send KDC and
kpasswd requests on UNIX domain sockets if any are listed in the kdc
and primary_kdc realm variables.

[ghudson@mit.edu: combined several commits; simplified client side by
treating UNIX domain socket entries like module-generated addresses;
edited commit message]

ticket: 9155