Make SASL work over UNIX domain sockets

Make SASL work over UNIX domain sockets

The addrToString methods were not coping with UNIX domain sockets
which have no normal host+port address. Hardcode special handling
for these so that SASL routines can work over UNIX sockets. Also
fix up SSF logic in remote client so that it presumes that a UNIX
socket is secure

* daemon/remote.c: Fix addrToString for UNIX sockets.
* src/remote/remote_driver.c: Fix addrToString for UNIX sockets
  and fix SSF logic to work for TLS + UNIX sockets in the same
  manner