[syslog-ng] allow setting up unix named sockets for listening

[Patrick Hemmer]

When troubleshooting issues with our production syslog servers, its 
often not possible to deliberately duplicate discovered issues as it 
then affects production. Something which would be extremely useful in 
this case is if we could configure syslog-ng to set up a unix socket as 
a listener. Then whenever clients connect logs can be sent to the client.

Syslog-ng currently has unix-stream and unix-dgram destinations, but 
they act as clients, only sending to already created sockets. There 
could be other uses for this, basically allowing clients to connect 
whenever they want and receive the logs. Kind of like a pipe 
destination, but it works a bit cleaner since if no client is connected, 
syslog-ng wouldnt have to buffer and drop messages (and it would allow 
multiple clients).

Fairly low priority, but just something that I thought would be useful.

-Patrick