Allow shared_port to fall back to local directory. #4465

In some cases, we don't actually want to use the abstract namespaces
- for example, if a client (condor_ssh_to_job) wants to register a
socket with the shared port.  Hence, we allow fallback to the
old style DAEMON_SOCKET_DIR.

On non-Linux platforms, if DAEMON_SOCKET_DIR is more than 100 chars,
then the fallback may fail.