better handling of local addresses

 - policy.c: SelectConnectionTypeIndex(): actually copy out
   the selected address when it matches the the incoming
   address; when tentatively selecting an address, only
   tentatively select the first valid address.  Place notes
   on fallback to incoming address when no valid addresses
   (should probably only be done when list is empty).

 - xdmcp.c: NetworkAddressToHostname(): if the address being
   converted is local to the XDM host, always return the
   string "localhost".  This result is used for ACL, so
   using the name "localhost" in the ACL will handle all
   local addresses.

 - xdmcp.c: NetworkAddressToName(): if the address being
   used for connection will be a local address, never prefix
   the display name with host or number.